submodule--helper: get remote names from any repository

`get_default_remote()` retrieves the name of a remote by resolving the
refs from of the current repository's ref store.

Thus in order to use it for retrieving the remote name of a submodule,
we have to start a new subprocess which runs from the submodule
directory.

Let's instead introduce a function called `repo_get_default_remote()`
which takes any repository object and retrieves the remote accordingly.

`get_default_remote()` is then defined as a call to
`repo_get_default_remote()` with 'the_repository' passed to it.

Now that we have `repo_get_default_remote()`, we no longer have to start
a subprocess that called `submodule--helper get-default-remote` from
within the submodule directory.

So let's make a function called `get_default_remote_submodule()` which
takes a submodule path, and returns the default remote for that
submodule, all within the same process.

We can now use this function to save an unnecessary subprocess spawn in
`sync_submodule()`, and also in a subsequent patch, which will require
this functionality.

Mentored-by: Christian Couder <christian.couder@gmail.com>
Mentored-by: Shourya Shukla <periperidip@gmail.com>
Helped-by: Glen Choo <chooglen@google.com>
Signed-off-by: Atharva Raykar <raykar.ath@gmail.com>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Glen Choo <chooglen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/submodule--helper.c

@@ -31,11 +31,13 @@
 typedef void (*each_submodule_fn)(const struct cache_entry *list_item,
 				  void *cb_data);
 
-static char *get_default_remote(void)
+static char *repo_get_default_remote(struct repository *repo)
 {
 	char *dest = NULL, *ret;
 	struct strbuf sb = STRBUF_INIT;
-	const char *refname = resolve_ref_unsafe("HEAD", 0, NULL, NULL);
+	struct ref_store *store = get_main_ref_store(repo);
+	const char *refname = refs_resolve_ref_unsafe(store, "HEAD", 0, NULL,
+						      NULL);
 
 	if (!refname)
 		die(_("No such ref: %s"), "HEAD");
@@ -48,7 +50,7 @@ static char *get_default_remote(void)
 		die(_("Expecting a full ref name, got %s"), refname);
 
 	strbuf_addf(&sb, "branch.%s.remote", refname);
-	if (git_config_get_string(sb.buf, &dest))
+	if (repo_config_get_string(repo, sb.buf, &dest))
 		ret = xstrdup("origin");
 	else
 		ret = dest;
@@ -57,6 +59,19 @@ static char *get_default_remote(void)
 	return ret;
 }
 
+static char *get_default_remote_submodule(const char *module_path)
+{
+	struct repository subrepo;
+
+	repo_submodule_init(&subrepo, the_repository, module_path, null_oid());
+	return repo_get_default_remote(&subrepo);
+}
+
+static char *get_default_remote(void)
+{
+	return repo_get_default_remote(the_repository);
+}
+
 static int print_default_remote(int argc, const char **argv, const char *prefix)
 {
 	char *remote;
@@ -1343,9 +1358,8 @@ static void sync_submodule(const char *path, const char *prefix,
 {
 	const struct submodule *sub;
 	char *remote_key = NULL;
-	char *sub_origin_url, *super_config_url, *displaypath;
+	char *sub_origin_url, *super_config_url, *displaypath, *default_remote;
 	struct strbuf sb = STRBUF_INIT;
-	struct child_process cp = CHILD_PROCESS_INIT;
 	char *sub_config_path = NULL;
 
 	if (!is_submodule_active(the_repository, path))
@@ -1384,21 +1398,15 @@ static void sync_submodule(const char *path, const char *prefix,
 	if (!is_submodule_populated_gently(path, NULL))
 		goto cleanup;
 
-	prepare_submodule_repo_env(&cp.env_array);
-	cp.git_cmd = 1;
-	cp.dir = path;
-	strvec_pushl(&cp.args, "submodule--helper",
-		     "print-default-remote", NULL);
-
 	strbuf_reset(&sb);
-	if (capture_command(&cp, &sb, 0))
+	default_remote = get_default_remote_submodule(path);
+	if (!default_remote)
 		die(_("failed to get the default remote for submodule '%s'"),
 		      path);
 
-	strbuf_strip_suffix(&sb, "\n");
-	remote_key = xstrfmt("remote.%s.url", sb.buf);
+	remote_key = xstrfmt("remote.%s.url", default_remote);
+	free(default_remote);
 
-	strbuf_reset(&sb);
 	submodule_to_gitdir(&sb, path);
 	strbuf_addstr(&sb, "/config");