diff --git a/Makefile b/Makefile
index 317be3c373..ccb896e812 100644
--- a/Makefile
+++ b/Makefile
@@ -153,7 +153,7 @@ PROGRAMS = \
 	git-upload-pack$X git-verify-pack$X git-write-tree$X \
 	git-update-ref$X git-symbolic-ref$X git-check-ref-format$X \
 	git-name-rev$X git-pack-redundant$X git-repo-config$X git-var$X \
-	git-describe$X
+	git-describe$X git-merge-tree$X
 
 # what 'all' will build and 'install' will install, in gitexecdir
 ALL_PROGRAMS = $(PROGRAMS) $(SIMPLE_PROGRAMS) $(SCRIPTS)
diff --git a/merge-tree.c b/merge-tree.c
new file mode 100644
index 0000000000..768d83a7c2
--- /dev/null
+++ b/merge-tree.c
@@ -0,0 +1,272 @@
+#include "cache.h"
+#include "diff.h"
+
+static const char merge_tree_usage[] = "git-merge-tree <base-tree> <branch1> <branch2>";
+static int resolve_directories = 1;
+
+static void merge_trees(struct tree_desc t[3], const char *base);
+
+static void *fill_tree_descriptor(struct tree_desc *desc, const unsigned char *sha1)
+{
+	unsigned long size = 0;
+	void *buf = NULL;
+
+	if (sha1) {
+		buf = read_object_with_reference(sha1, "tree", &size, NULL);
+		if (!buf)
+			die("unable to read tree %s", sha1_to_hex(sha1));
+	}
+	desc->size = size;
+	desc->buf = buf;
+	return buf;
+}
+
+struct name_entry {
+	const unsigned char *sha1;
+	const char *path;
+	unsigned int mode;
+	int pathlen;
+};
+
+static void entry_clear(struct name_entry *a)
+{
+	memset(a, 0, sizeof(*a));
+}
+
+static int entry_compare(struct name_entry *a, struct name_entry *b)
+{
+	return base_name_compare(
+			a->path, a->pathlen, a->mode,
+			b->path, b->pathlen, b->mode);
+}
+
+static void entry_extract(struct tree_desc *t, struct name_entry *a)
+{
+	a->sha1 = tree_entry_extract(t, &a->path, &a->mode);
+	a->pathlen = strlen(a->path);
+}
+
+/* An empty entry never compares same, not even to another empty entry */
+static int same_entry(struct name_entry *a, struct name_entry *b)
+{
+	return	a->sha1 &&
+		b->sha1 &&
+		!memcmp(a->sha1, b->sha1, 20) &&
+		a->mode == b->mode;
+}
+
+static const char *sha1_to_hex_zero(const unsigned char *sha1)
+{
+	if (sha1)
+		return sha1_to_hex(sha1);
+	return "0000000000000000000000000000000000000000";
+}
+
+static void resolve(const char *base, struct name_entry *branch1, struct name_entry *result)
+{
+	char branch1_sha1[50];
+
+	/* If it's already branch1, don't bother showing it */
+	if (!branch1)
+		return;
+	memcpy(branch1_sha1, sha1_to_hex_zero(branch1->sha1), 41);
+
+	printf("0 %06o->%06o %s->%s %s%s\n",
+		branch1->mode, result->mode,
+		branch1_sha1, sha1_to_hex_zero(result->sha1),
+		base, result->path);
+}
+
+static int unresolved_directory(const char *base, struct name_entry n[3])
+{
+	int baselen;
+	char *newbase;
+	struct name_entry *p;
+	struct tree_desc t[3];
+	void *buf0, *buf1, *buf2;
+
+	if (!resolve_directories)
+		return 0;
+	p = n;
+	if (!p->mode) {
+		p++;
+		if (!p->mode)
+			p++;
+	}
+	if (!S_ISDIR(p->mode))
+		return 0;
+	baselen = strlen(base);
+	newbase = xmalloc(baselen + p->pathlen + 2);
+	memcpy(newbase, base, baselen);
+	memcpy(newbase + baselen, p->path, p->pathlen);
+	memcpy(newbase + baselen + p->pathlen, "/", 2);
+
+	buf0 = fill_tree_descriptor(t+0, n[0].sha1);
+	buf1 = fill_tree_descriptor(t+1, n[1].sha1);
+	buf2 = fill_tree_descriptor(t+2, n[2].sha1);
+	merge_trees(t, newbase);
+
+	free(buf0);
+	free(buf1);
+	free(buf2);
+	free(newbase);
+	return 1;
+}
+
+static void unresolved(const char *base, struct name_entry n[3])
+{
+	if (unresolved_directory(base, n))
+		return;
+	if (n[0].sha1)
+		printf("1 %06o %s %s%s\n", n[0].mode, sha1_to_hex(n[0].sha1), base, n[0].path);
+	if (n[1].sha1)
+		printf("2 %06o %s %s%s\n", n[1].mode, sha1_to_hex(n[1].sha1), base, n[1].path);
+	if (n[2].sha1)
+		printf("3 %06o %s %s%s\n", n[2].mode, sha1_to_hex(n[2].sha1), base, n[2].path);
+}
+
+typedef void (*traverse_callback_t)(int n, unsigned long mask, struct name_entry *entry, const char *base);
+
+static void traverse_trees(int n, struct tree_desc *t, const char *base, traverse_callback_t callback)
+{
+	struct name_entry *entry = xmalloc(n*sizeof(*entry));
+
+	for (;;) {
+		struct name_entry entry[3];
+		unsigned long mask = 0;
+		int i, last;
+
+		last = -1;
+		for (i = 0; i < n; i++) {
+			if (!t[i].size)
+				continue;
+			entry_extract(t+i, entry+i);
+			if (last >= 0) {
+				int cmp = entry_compare(entry+i, entry+last);
+
+				/*
+				 * Is the new name bigger than the old one?
+				 * Ignore it
+				 */
+				if (cmp > 0)
+					continue;
+				/*
+				 * Is the new name smaller than the old one?
+				 * Ignore all old ones
+				 */
+				if (cmp < 0)
+					mask = 0;
+			}
+			mask |= 1ul << i;
+			last = i;
+		}
+		if (!mask)
+			break;
+
+		/*
+		 * Update the tree entries we've walked, and clear
+		 * all the unused name-entries.
+		 */
+		for (i = 0; i < n; i++) {
+			if (mask & (1ul << i)) {
+				update_tree_entry(t+i);
+				continue;
+			}
+			entry_clear(entry + i);
+		}
+		callback(n, mask, entry, base);
+	}
+	free(entry);
+}
+
+/*
+ * Merge two trees together (t[1] and t[2]), using a common base (t[0])
+ * as the origin.
+ *
+ * This walks the (sorted) trees in lock-step, checking every possible
+ * name. Note that directories automatically sort differently from other
+ * files (see "base_name_compare"), so you'll never see file/directory
+ * conflicts, because they won't ever compare the same.
+ *
+ * IOW, if a directory changes to a filename, it will automatically be
+ * seen as the directory going away, and the filename being created.
+ *
+ * Think of this as a three-way diff.
+ *
+ * The output will be either:
+ *  - successful merge
+ *	 "0 mode sha1 filename"
+ *    NOTE NOTE NOTE! FIXME! We really really need to walk the index
+ *    in parallel with this too!
+ *
+ *  - conflict:
+ *	"1 mode sha1 filename"
+ *	"2 mode sha1 filename"
+ *	"3 mode sha1 filename"
+ *    where not all of the 1/2/3 lines may exist, of course.
+ *
+ * The successful merge rules are the same as for the three-way merge
+ * in git-read-tree.
+ */
+static void threeway_callback(int n, unsigned long mask, struct name_entry *entry, const char *base)
+{
+	/* Same in both? */
+	if (same_entry(entry+1, entry+2)) {
+		if (entry[0].sha1) {
+			resolve(base, NULL, entry+1);
+			return;
+		}
+	}
+
+	if (same_entry(entry+0, entry+1)) {
+		if (entry[2].sha1 && !S_ISDIR(entry[2].mode)) {
+			resolve(base, entry+1, entry+2);
+			return;
+		}
+	}
+
+	if (same_entry(entry+0, entry+2)) {
+		if (entry[1].sha1 && !S_ISDIR(entry[1].mode)) {
+			resolve(base, NULL, entry+1);
+			return;
+		}
+	}
+
+	unresolved(base, entry);
+}
+
+static void merge_trees(struct tree_desc t[3], const char *base)
+{
+	traverse_trees(3, t, base, threeway_callback);
+}
+
+static void *get_tree_descriptor(struct tree_desc *desc, const char *rev)
+{
+	unsigned char sha1[20];
+	void *buf;
+
+	if (get_sha1(rev, sha1) < 0)
+		die("unknown rev %s", rev);
+	buf = fill_tree_descriptor(desc, sha1);
+	if (!buf)
+		die("%s is not a tree", rev);
+	return buf;
+}
+
+int main(int argc, char **argv)
+{
+	struct tree_desc t[3];
+	void *buf1, *buf2, *buf3;
+
+	if (argc < 4)
+		usage(merge_tree_usage);
+
+	buf1 = get_tree_descriptor(t+0, argv[1]);
+	buf2 = get_tree_descriptor(t+1, argv[2]);
+	buf3 = get_tree_descriptor(t+2, argv[3]);
+	merge_trees(t, "");
+	free(buf1);
+	free(buf2);
+	free(buf3);
+	return 0;
+}