submodule: add a print_config_from_gitmodules() helper

Add a new print_config_from_gitmodules() helper function to print values
from .gitmodules just like "git config -f .gitmodules" would.

This will be used by a new submodule--helper subcommand to be able to
access the .gitmodules file in a more controlled way.

Signed-off-by: Antonio Ospite <ao2@ao2.it>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

submodule-config.c

@@ -682,6 +682,31 @@ void submodule_free(struct repository *r)
 		submodule_cache_clear(r->submodule_cache);
 }
 
+static int config_print_callback(const char *var, const char *value, void *cb_data)
+{
+	char *wanted_key = cb_data;
+
+	if (!strcmp(wanted_key, var))
+		printf("%s\n", value);
+
+	return 0;
+}
+
+int print_config_from_gitmodules(struct repository *repo, const char *key)
+{
+	int ret;
+	char *store_key;
+
+	ret = git_config_parse_key(key, &store_key, NULL);
+	if (ret < 0)
+		return CONFIG_INVALID_KEY;
+
+	config_from_gitmodules(config_print_callback, repo, store_key);
+
+	free(store_key);
+	return 0;
+}
+
 struct fetch_config {
 	int *max_children;
 	int *recurse_submodules;

submodule-config.h

@@ -48,6 +48,7 @@ const struct submodule *submodule_from_path(struct repository *r,
 					    const struct object_id *commit_or_tree,
 					    const char *path);
 void submodule_free(struct repository *r);
+int print_config_from_gitmodules(struct repository *repo, const char *key);
 
 /*
  * Returns 0 if the name is syntactically acceptable as a submodule "name"