Fix 'git help help'

git help foo invokes man git-foo if foo is a git command, otherwise it
invokes man gitfoo. 'help' is not a git command, but the manual page is
called git-help, so add this special exception.

Signed-off-by: Miklos Vajna <vmiklos@frugalware.org>
Acked-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

help.c

@@ -555,7 +555,8 @@ static int is_git_command(const char *s)
 {
 	load_command_list();
 	return is_in_cmdlist(&main_cmds, s) ||
-		is_in_cmdlist(&other_cmds, s);
+		is_in_cmdlist(&other_cmds, s) ||
+		!strcmp(s, "help");
 }
 
 static const char *prepend(const char *prefix, const char *cmd)