submodules: load gitmodules file from commit sha1

teach submodules to load a '.gitmodules' file from a commit sha1.  This
enables the population of the submodule_cache to be based on the state
of the '.gitmodules' file from a particular commit.

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

submodule-config.c

@@ -379,9 +379,9 @@ static int parse_config(const char *var, const char *value, void *data)
 	return ret;
 }
 
-static int gitmodule_sha1_from_commit(const unsigned char *commit_sha1,
-				      unsigned char *gitmodules_sha1,
-				      struct strbuf *rev)
+int gitmodule_sha1_from_commit(const unsigned char *commit_sha1,
+			       unsigned char *gitmodules_sha1,
+			       struct strbuf *rev)
 {
 	int ret = 0;