submodule: add repo_read_gitmodules

Teach the repo object to be able to populate the submodule_cache by
reading the repository's gitmodules file.
Signed-off-by: NBrandon Williams <bmwill@google.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

submodule.c

 #include "cache.h"
+#include "repository.h"
 #include "config.h"
 #include "submodule-config.h"
 #include "submodule.h"
@@ -255,6 +256,20 @@ void gitmodules_config(void)
 	}
 }
 
+static int gitmodules_cb(const char *var, const char *value, void *data)
+{
+	struct repository *repo = data;
+	return submodule_config_option(repo, var, value);
+}
+
+void repo_read_gitmodules(struct repository *repo)
+{
+	char *gitmodules_path = repo_worktree_path(repo, ".gitmodules");
+
+	git_config_from_file(gitmodules_cb, gitmodules_path, repo);
+	free(gitmodules_path);
+}
+
 void gitmodules_config_sha1(const unsigned char *commit_sha1)
 {
 	struct strbuf rev = STRBUF_INIT;

submodule.h

 #ifndef SUBMODULE_H
 #define SUBMODULE_H
 
+struct repository;
 struct diff_options;
 struct argv_array;
 struct oid_array;
@@ -46,6 +47,7 @@ int option_parse_recurse_submodules_worktree_updater(const struct option *opt,
 						     const char *arg, int unset);
 void load_submodule_cache(void);
 extern void gitmodules_config(void);
+extern void repo_read_gitmodules(struct repository *repo);
 extern void gitmodules_config_sha1(const unsigned char *commit_sha1);
 extern int is_submodule_initialized(const char *path);
 /*