submodules: add helper to determine if a submodule is initialized

Add the `is_submodule_initialized()` helper function to submodules.c.
`is_submodule_initialized()` performs a check to determine if the
submodule at the given path has been initialized.

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

submodule.c

@@ -198,6 +198,29 @@ void gitmodules_config(void)
 	}
 }
 
+/*
+ * Determine if a submodule has been initialized at a given 'path'
+ */
+int is_submodule_initialized(const char *path)
+{
+	int ret = 0;
+	const struct submodule *module = NULL;
+
+	module = submodule_from_path(null_sha1, path);
+
+	if (module) {
+		char *key = xstrfmt("submodule.%s.url", module->name);
+		char *value = NULL;
+
+		ret = !git_config_get_string(key, &value);
+
+		free(value);
+		free(key);
+	}
+
+	return ret;
+}
+
 /*
  * Determine if a submodule has been populated 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 int is_submodule_initialized(const char *path);
 extern int is_submodule_populated(const char *path);
 int parse_submodule_update_strategy(const char *value,
 		struct submodule_update_strategy *dst);