name-rev: allow converting the exact object name at the tip of a ref

"git name-rev" is supposed to convert given object names into
strings that name the same objects based on refs, that can be fed to
"git rev-parse" to get the same object names back, so the output for
the commit object v1.8.3^0 (i.e. the commit tagged as v1.8.3)

    $ git rev-parse v1.8.3 v1.8.3^0 | git name-rev --stdin
    8af06057d0
    edca415256 (tags/v1.8.3^0)

has to have "^0" at the end, as "edca41" is a commit, not the tag
that references it.  But we do not get anything for the tag object
(8af0605) itself.

This is because the command however did not bother to see if the
object is at the tip of some ref, and failed to convert a tag
object.

Teach it to show this instead:

    $ git rev-parse v1.8.3 v1.8.3^0 | git name-rev --stdin
    8af06057d0 (tags/v1.8.3)
    edca415256 (tags/v1.8.3^0)

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/name-rev.c

@@ -4,6 +4,7 @@
 #include "tag.h"
 #include "refs.h"
 #include "parse-options.h"
+#include "sha1-lookup.h"
 
 #define CUTOFF_DATE_SLOP 86400 /* one day */
 
@@ -113,6 +114,34 @@ struct name_ref_data {
 	const char *ref_filter;
 };
 
+static struct tip_table {
+	struct tip_table_entry {
+		unsigned char sha1[20];
+		const char *refname;
+	} *table;
+	int nr;
+	int alloc;
+	int sorted;
+} tip_table;
+
+static void add_to_tip_table(const unsigned char *sha1, const char *refname,
+			     int shorten_unambiguous)
+{
+	refname = name_ref_abbrev(refname, shorten_unambiguous);
+
+	ALLOC_GROW(tip_table.table, tip_table.nr + 1, tip_table.alloc);
+	hashcpy(tip_table.table[tip_table.nr].sha1, sha1);
+	tip_table.table[tip_table.nr].refname = xstrdup(refname);
+	tip_table.nr++;
+	tip_table.sorted = 0;
+}
+
+static int tipcmp(const void *a_, const void *b_)
+{
+	const struct tip_table_entry *a = a_, *b = b_;
+	return hashcmp(a->sha1, b->sha1);
+}
+
 static int name_ref(const char *path, const unsigned char *sha1, int flags, void *cb_data)
 {
 	struct object *o = parse_object(sha1);
@@ -135,6 +164,8 @@ static int name_ref(const char *path, const unsigned char *sha1, int flags, void
 		}
 	}
 
+	add_to_tip_table(sha1, path, can_abbreviate_output);
+
 	while (o && o->type == OBJ_TAG) {
 		struct tag *t = (struct tag *) o;
 		if (!t->tagged)
@@ -151,6 +182,32 @@ static int name_ref(const char *path, const unsigned char *sha1, int flags, void
 	return 0;
 }
 
+static const unsigned char *nth_tip_table_ent(size_t ix, void *table_)
+{
+	struct tip_table_entry *table = table_;
+	return table[ix].sha1;
+}
+
+static const char *get_exact_ref_match(const struct object *o)
+{
+	int found;
+
+	if (!tip_table.table || !tip_table.nr)
+		return NULL;
+
+	if (!tip_table.sorted) {
+		qsort(tip_table.table, tip_table.nr, sizeof(*tip_table.table),
+		      tipcmp);
+		tip_table.sorted = 1;
+	}
+
+	found = sha1_pos(o->sha1, tip_table.table, tip_table.nr,
+			 nth_tip_table_ent);
+	if (0 <= found)
+		return tip_table.table[found].refname;
+	return NULL;
+}
+
 /* returns a static buffer */
 static const char *get_rev_name(const struct object *o)
 {
@@ -159,7 +216,7 @@ static const char *get_rev_name(const struct object *o)
 	struct commit *c;
 
 	if (o->type != OBJ_COMMIT)
-		return NULL;
+		return get_exact_ref_match(o);
 	c = (struct commit *) o;
 	n = c->util;
 	if (!n)