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>

cache.h

@@ -1693,6 +1693,8 @@ extern int git_default_config(const char *, const char *, void *);
 extern int git_config_from_file(config_fn_t fn, const char *, void *);
 extern int git_config_from_mem(config_fn_t fn, const enum config_origin_type,
 					const char *name, const char *buf, size_t len, void *data);
+extern int git_config_from_blob_sha1(config_fn_t fn, const char *name,
+				     const unsigned char *sha1, void *data);
 extern void git_config_push_parameter(const char *text);
 extern int git_config_from_parameters(config_fn_t fn, void *data);
 extern void git_config(config_fn_t fn, void *);

config.c

@@ -1214,10 +1214,10 @@ int git_config_from_mem(config_fn_t fn, const enum config_origin_type origin_typ
 	return do_config_from(&top, fn, data);
 }
 
-static int git_config_from_blob_sha1(config_fn_t fn,
+int git_config_from_blob_sha1(config_fn_t fn,
-				     const char *name,
+			      const char *name,
-				     const unsigned char *sha1,
+			      const unsigned char *sha1,
-				     void *data)
+			      void *data)
 {
 	enum object_type type;
 	char *buf;

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,
+int gitmodule_sha1_from_commit(const unsigned char *commit_sha1,
-				      unsigned char *gitmodules_sha1,
+			       unsigned char *gitmodules_sha1,
-				      struct strbuf *rev)
+			       struct strbuf *rev)
 {
 	int ret = 0;
 

submodule-config.h

@@ -29,6 +29,9 @@ const struct submodule *submodule_from_name(const unsigned char *commit_sha1,
 		const char *name);
 const struct submodule *submodule_from_path(const unsigned char *commit_sha1,
 		const char *path);
+extern int gitmodule_sha1_from_commit(const unsigned char *commit_sha1,
+				      unsigned char *gitmodules_sha1,
+				      struct strbuf *rev);
 void submodule_free(void);
 
 #endif /* SUBMODULE_CONFIG_H */

submodule.c

@@ -198,6 +198,18 @@ void gitmodules_config(void)
 	}
 }
 
+void gitmodules_config_sha1(const unsigned char *commit_sha1)
+{
+	struct strbuf rev = STRBUF_INIT;
+	unsigned char sha1[20];
+
+	if (gitmodule_sha1_from_commit(commit_sha1, sha1, &rev)) {
+		git_config_from_blob_sha1(submodule_config, rev.buf,
+					  sha1, NULL);
+	}
+	strbuf_release(&rev);
+}
+
 /*
  * Determine if a submodule has been initialized at a given 'path'
  */

submodule.h

@@ -37,6 +37,7 @@ void set_diffopt_flags_from_submodule_config(struct diff_options *diffopt,
 		const char *path);
 int submodule_config(const char *var, const char *value, void *cb);
 void gitmodules_config(void);
+extern void gitmodules_config_sha1(const unsigned char *commit_sha1);
 extern int is_submodule_initialized(const char *path);
 extern int is_submodule_populated(const char *path);
 int parse_submodule_update_strategy(const char *value,