[PATCH] sha1_name: avoid unnecessary sha1 lookup in find_unique_abbrev

[Mike Hommey <mh@glandium.org>]

An example where this happens is when doing an ls-tree on a tree that
contains a commit link. In that case, find_unique_abbrev is called
to get a non-abbreviated hex sha1, but still, a lookup is done as
to whether the sha1 is in the repository (which ends up looking for
a loose object in .git/objects), while the result of that lookup is
not used when returning a non-abbreviated hex sha1.

Signed-off-by: Mike Hommey <mh@glandium.org>
---
 sha1_name.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

This hit me on a corner case, where I kind of abuse commit links and
have a tree with tens of thousands of them.

Without the patch:
  $ time git ls-tree -r HEAD | wc -l
  114987

  real	0m4.412s
  user	0m1.980s
  sys	0m2.480s

With the patch:
  $ time git ls-tree -r HEAD | wc -l
  114987

  real	0m0.205s
  user	0m0.196s
  sys	0m0.012s


diff --git a/sha1_name.c b/sha1_name.c
index 5b004f5..cb88170 100644
--- a/sha1_name.c
+++ b/sha1_name.c
@@ -372,10 +372,10 @@ const char *find_unique_abbrev(const unsigned char *sha1, int len)
 	int status, exists;
 	static char hex[41];
 
-	exists = has_sha1_file(sha1);
 	memcpy(hex, sha1_to_hex(sha1), 40);
 	if (len == 40 || !len)
 		return hex;
+	exists = has_sha1_file(sha1);
 	while (len < 40) {
 		unsigned char sha1_ret[20];
 		status = get_short_sha1(hex, len, sha1_ret, GET_SHA1_QUIETLY);
-- 
2.1.1