Allow to control where the replace refs are looked for

It can be useful to have grafts or replace refs for specific use-cases while
keeping the default "view" of the repository pristine (or with a different
set of grafts/replace refs).

It is possible to use a different graft file with GIT_GRAFT_FILE, but while
replace refs are more powerful, they don't have an equivalent override.

Add a GIT_REPLACE_REF_BASE environment variable to control where git is
going to look for replace refs.

Signed-off-by: Mike Hommey <mh@glandium.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin/replace.c b/builtin/replace.c
index 54bf01a..b5b7139 100644
--- a/builtin/replace.c
+++ b/builtin/replace.c
@@ -104,9 +104,9 @@
 			continue;
 		}
 		full_hex = sha1_to_hex(sha1);
-		snprintf(ref, sizeof(ref), "refs/replace/%s", full_hex);
+		snprintf(ref, sizeof(ref), "%s%s", git_replace_ref_base, full_hex);
 		/* read_ref() may reuse the buffer */
-		full_hex = ref + strlen("refs/replace/");
+		full_hex = ref + strlen(git_replace_ref_base);
 		if (read_ref(ref, sha1)) {
 			error("replace ref '%s' not found.", full_hex);
 			had_error = 1;
@@ -134,7 +134,7 @@
 			    int force)
 {
 	if (snprintf(ref, ref_size,
-		     "refs/replace/%s",
+		     "%s%s", git_replace_ref_base,
 		     sha1_to_hex(object)) > ref_size - 1)
 		die("replace ref name too long: %.*s...", 50, ref);
 	if (check_refname_format(ref, 0))