Add stupid "git export" thing, which can export a git archive

as a set of patches and commentary.

You'd want something like this if you are tracking a git archive
in another SCM format. Notably, we want something like that for
BK users.

Makefile

@@ -15,7 +15,7 @@ AR=ar
 
 PROG=   update-cache show-diff init-db write-tree read-tree commit-tree \
 	cat-file fsck-cache checkout-cache diff-tree rev-tree show-files \
-	check-files ls-tree merge-base merge-cache unpack-file
+	check-files ls-tree merge-base merge-cache unpack-file git-export
 
 all: $(PROG)
 
@@ -81,6 +81,9 @@ merge-cache: merge-cache.o $(LIB_FILE)
 unpack-file: unpack-file.o $(LIB_FILE)
 	$(CC) $(CFLAGS) -o unpack-file unpack-file.o $(LIBS)
 
+git-export: git-export.o $(LIB_FILE)
+	$(CC) $(CFLAGS) -o git-export git-export.o $(LIBS)
+
 blob.o: $(LIB_H)
 cat-file.o: $(LIB_H)
 check-files.o: $(LIB_H)
@@ -89,6 +92,7 @@ commit.o: $(LIB_H)
 commit-tree.o: $(LIB_H)
 diff-tree.o: $(LIB_H)
 fsck-cache.o: $(LIB_H)
+git_export.o: $(LIB_H)
 init-db.o: $(LIB_H)
 ls-tree.o: $(LIB_H)
 merge-base.o: $(LIB_H)

git-export.c

@@ -0,0 +1,81 @@
+#include "cache.h"
+#include "commit.h"
+
+/*
+ * Show one commit
+ */
+void show_commit(struct commit *commit)
+{
+	char cmdline[400];
+	char hex[100];
+
+	strcpy(hex, sha1_to_hex(commit->object.sha1));
+	printf("Id: %s\n", hex);
+	fflush(NULL);
+	sprintf(cmdline, "cat-file commit %s", hex);
+	system(cmdline);
+	if (commit->parents) {
+		char *against = sha1_to_hex(commit->parents->item->object.sha1);
+		printf("\n\n======== diff against %s ========\n", against);
+		fflush(NULL);
+		sprintf(cmdline, "git diff %s %s", against, hex);
+		system(cmdline);
+	}
+	printf("======== end ========\n\n");
+}
+
+/*
+ * Show all unseen commits, depth-first
+ */
+void show_unseen(struct commit *top)
+{
+	struct commit_list *parents;
+
+	if (top->object.flags & 2)
+		return;
+	top->object.flags |= 2;
+	parents = top->parents;
+	while (parents) {
+		show_unseen(parents->item);
+		parents = parents->next;
+	}
+	show_commit(top);
+}
+
+void export(struct commit *top, struct commit *base)
+{
+	mark_reachable(&top->object, 1);
+	if (base)
+		mark_reachable(&base->object, 2);
+	show_unseen(top);
+}
+
+struct commit *get_commit(unsigned char *sha1)
+{
+	struct commit *commit = lookup_commit(sha1);
+	if (!commit->object.parsed) {
+		struct commit_list *parents;
+
+		if (parse_commit(commit) < 0)
+			die("unable to parse commit %s", sha1_to_hex(sha1));
+		parents = commit->parents;
+		while (parents) {
+			get_commit(parents->item->object.sha1);
+			parents = parents->next;
+		}
+	}
+	return commit;
+}
+
+int main(int argc, char **argv)
+{
+	unsigned char base_sha1[20];
+	unsigned char top_sha1[20];
+
+	if (argc < 2 || argc > 4 ||
+	    get_sha1_hex(argv[1], top_sha1) ||
+	    (argc == 3 && get_sha1_hex(argv[2], base_sha1)))
+		usage("git-export top [base]");
+	export(get_commit(top_sha1), argc==3 ? get_commit(base_sha1) : NULL);
+	return 0;
+}