builtin/help.c: speed up is_git_command() by checking for builtin commands first

Since 2dce956 is_git_command() is a bit slow as it does file I/O in
the call to list_commands_in_dir(). Avoid the file I/O by adding an
early check for the builtin commands.

Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/technical/api-builtin.txt

@@ -14,8 +14,8 @@ Git:
 
 . Add the external declaration for the function to `builtin.h`.
 
-. Add the command to `commands[]` table in `handle_builtin()`,
-  defined in `git.c`.  The entry should look like:
+. Add the command to the `commands[]` table defined in `git.c`.
+  The entry should look like:
 
 	{ "foo", cmd_foo, <options> },
 +

builtin.h

@@ -27,6 +27,8 @@ extern int fmt_merge_msg(struct strbuf *in, struct strbuf *out,
 
 extern int textconv_object(const char *path, unsigned mode, const unsigned char *sha1, int sha1_valid, char **buf, unsigned long *buf_size);
 
+extern int is_builtin(const char *s);
+
 extern int cmd_add(int argc, const char **argv, const char *prefix);
 extern int cmd_annotate(int argc, const char **argv, const char *prefix);
 extern int cmd_apply(int argc, const char **argv, const char *prefix);

builtin/help.c

@@ -288,6 +288,9 @@ static struct cmdnames main_cmds, other_cmds;
 
 static int is_git_command(const char *s)
 {
+	if (is_builtin(s))
+		return 1;
+
 	load_command_list("git-", &main_cmds, &other_cmds);
 	return is_in_cmdlist(&main_cmds, s) ||
 		is_in_cmdlist(&other_cmds, s);

git.c

@@ -332,9 +332,6 @@ static int run_builtin(struct cmd_struct *p, int argc, const char **argv)
 	return 0;
 }
 
-static void handle_builtin(int argc, const char **argv)
-{
-	const char *cmd = argv[0];
 static struct cmd_struct commands[] = {
 	{ "add", cmd_add, RUN_SETUP | NEED_WORK_TREE },
 	{ "annotate", cmd_annotate, RUN_SETUP },
@@ -450,6 +447,21 @@ static void handle_builtin(int argc, const char **argv)
 	{ "whatchanged", cmd_whatchanged, RUN_SETUP },
 	{ "write-tree", cmd_write_tree, RUN_SETUP },
 };
+
+int is_builtin(const char *s)
+{
+	int i;
+	for (i = 0; i < ARRAY_SIZE(commands); i++) {
+		struct cmd_struct *p = commands+i;
+		if (!strcmp(s, p->cmd))
+			return 1;
+	}
+	return 0;
+}
+
+static void handle_builtin(int argc, const char **argv)
+{
+	const char *cmd = argv[0];
 	int i;
 	static const char ext[] = STRIP_EXTENSION;