feat(tag): Add submodule tagging

Signed-off-by: dark0dave <dark0dave@mykolab.com>

submodule-config.c

@@ -91,6 +91,7 @@ static void free_one_config(struct submodule_entry *entry)
 	free((void *) entry->config->path);
 	free((void *) entry->config->name);
 	free((void *) entry->config->branch);
+	free((void *) entry->config->tag);
 	free((void *) entry->config->update_strategy.command);
 	free(entry->config);
 }
@@ -413,6 +414,7 @@ static struct submodule *lookup_or_create_by_name(struct submodule_cache *cache,
 	submodule->fetch_recurse = RECURSE_SUBMODULES_NONE;
 	submodule->ignore = NULL;
 	submodule->branch = NULL;
+	submodule->tag = NULL;
 	submodule->recommend_shallow = -1;
 
 	oidcpy(&submodule->gitmodules_oid, gitmodules_oid);
@@ -658,13 +660,25 @@ static int parse_config(const char *var, const char *value,
 	} else if (!strcmp(item.buf, "branch")) {
 		if (!value)
 			ret = config_error_nonbool(var);
+		else if (submodule->branch && submodule->tag)
+			die(_("can not specify both tag and branch '%s'"), var);
 		else if (!me->overwrite && submodule->branch)
 			warn_multiple_config(me->treeish_name, submodule->name,
 					     "branch");
 		else {
 			free((void *)submodule->branch);
 			submodule->branch = xstrdup(value);
 		}
+	} else if (!strcmp(item.buf, "tag")) {
+		if (!value)
+			ret = config_error_nonbool(var);
+		else if (!me->overwrite && submodule->tag)
+			warn_multiple_config(me->treeish_name, submodule->name,
+					     "tag");
+		else {
+			free((void *)submodule->tag);
+			submodule->tag = xstrdup(value);
+		}
 	}
 
 	strbuf_release(&name);

submodule-config.h

@@ -38,6 +38,7 @@ struct submodule {
 	enum submodule_recurse_mode fetch_recurse;
 	const char *ignore;
 	const char *branch;
+	const char *tag;
 	struct submodule_update_strategy update_strategy;
 	/* the object id of the responsible .gitmodules file */
 	struct object_id gitmodules_oid;