submodule: merge repo_read_gitmodules and gitmodules_config

Since 69aba5329 (submodule: add repo_read_gitmodules) there have been
two ways to load a repository's .gitmodules file:
'repo_read_gitmodules()' is used if you have a repository object you are
working with or 'gitmodules_config()' if you are implicitly working with
'the_repository'.  Merge the logic of these two functions to remove
duplicate code.

In addition, 'repo_read_gitmodules()' can segfault by passing in a NULL
pointer to 'git_config_from_file()' if a repository doesn't have a
worktree.  Instead check for the existence of a worktree before
attempting to load the .gitmodules file.

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

submodule.c

@@ -230,23 +230,6 @@ void load_submodule_cache(void)
 	git_config(submodule_config, NULL);
 }
 
-void gitmodules_config(void)
-{
-	const char *work_tree = get_git_work_tree();
-	if (work_tree) {
-		struct strbuf gitmodules_path = STRBUF_INIT;
-		strbuf_addstr(&gitmodules_path, work_tree);
-		strbuf_addstr(&gitmodules_path, "/" GITMODULES_FILE);
-		if (read_cache() < 0)
-			die("index file corrupt");
-
-		if (!is_gitmodules_unmerged(&the_index))
-			git_config_from_file(git_modules_config,
-				gitmodules_path.buf, NULL);
-		strbuf_release(&gitmodules_path);
-	}
-}
-
 static int gitmodules_cb(const char *var, const char *value, void *data)
 {
 	struct repository *repo = data;
@@ -255,10 +238,24 @@ static int gitmodules_cb(const char *var, const char *value, void *data)
 
 void repo_read_gitmodules(struct repository *repo)
 {
-	char *gitmodules_path = repo_worktree_path(repo, GITMODULES_FILE);
+	if (repo->worktree) {
+		char *gitmodules;
+
+		if (repo_read_index(repo) < 0)
+			return;
 
-	git_config_from_file(gitmodules_cb, gitmodules_path, repo);
-	free(gitmodules_path);
+		gitmodules = repo_worktree_path(repo, GITMODULES_FILE);
+
+		if (!is_gitmodules_unmerged(repo->index))
+			git_config_from_file(gitmodules_cb, gitmodules, repo);
+
+		free(gitmodules);
+	}
+}
+
+void gitmodules_config(void)
+{
+	repo_read_gitmodules(the_repository);
 }
 
 void gitmodules_config_sha1(const unsigned char *commit_sha1)