[PATCH v4 11/18] sha1_name.c: teach get_short_sha1() a commit-only option

[Junio C Hamano <gitster@pobox.com>]

When the caller knows that the parameter is meant to name a commit,
e.g. "56789a" in describe name "v1.2.3-4-g56789a", pass that as a
hint so that lower level can use it to disambiguate objects when
there is only one commit whose name begins with 56789a even if there
are objects of other types whose names share the same prefix.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 cache.h     | 1 +
 sha1_name.c | 9 +++++++++
 2 files changed, 10 insertions(+)

diff --git a/cache.h b/cache.h
index fd5b7f6..29f0b2a 100644
--- a/cache.h
+++ b/cache.h
@@ -812,6 +812,7 @@ struct object_context {
 };
 
 #define GET_SHA1_QUIETLY 01
+#define GET_SHA1_COMMIT 02
 
 extern int get_sha1(const char *str, unsigned char *sha1);
 extern void die_on_misspelt_object_name(const char *name, const char *prefix);
diff --git a/sha1_name.c b/sha1_name.c
index 01cce9f..be15d94 100644
--- a/sha1_name.c
+++ b/sha1_name.c
@@ -218,6 +218,12 @@ static int finish_object_disambiguation(struct disambiguate_state *ds,
 	return 0;
 }
 
+static int disambiguate_commit_only(const unsigned char *sha1, void *cb_data_unused)
+{
+	int kind = sha1_object_info(sha1, NULL);
+	return kind == OBJ_COMMIT;
+}
+
 static int get_short_sha1(const char *name, int len, unsigned char *sha1,
 			  unsigned flags)
 {
@@ -253,6 +259,9 @@ static int get_short_sha1(const char *name, int len, unsigned char *sha1,
 	prepare_alt_odb();
 
 	memset(&ds, 0, sizeof(ds));
+	if (flags & GET_SHA1_COMMIT)
+		ds.fn = disambiguate_commit_only;
+
 	find_short_object_filename(len, hex_pfx, &ds);
 	find_short_packed_object(len, bin_pfx, &ds);
 	status = finish_object_disambiguation(&ds, sha1);
-- 
1.7.11.1.212.g52fe12e