pathspec: avoid the need of "--" when wildcard is used

When "--" is lacking from the command line and a command can take
both revs and paths, the idea is if an argument can be seen as both
an extended SHA-1 and a path, then "--" is required or git refuses
to continue. It's currently implemented as:

 (1) if an argument is rev, then it must not exist in worktree

 (2) else, it must exist in worktree

 (3) else, "--" is required.

These rules work for literal paths, but when non-literal pathspec is
involved, it almost always requires the user to add "--" because it
fails (2) and (1) is really rarely met (take "*.c" for example, (1)
is met if there is a ref named "*.c").

This patch modifies the rules a bit by considering any valid (*)
wildcard pathspec "exist in worktree". The rules become:

 (1) if an arg is a rev, then it must either exist in worktree or
     not be a valid wildcard pathspec.

 (2) else, it either exists in worktree or is a wildcard pathspec

 (3) else, "--" is required.

With the new rules, "--" is not needed most of the time when
wildcard pathspec is involved.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

setup.c

@@ -140,7 +140,9 @@ int check_filename(const char *prefix, const char *arg)
 		if (arg[2] == '\0') /* ":/" is root dir, always exists */
 			return 1;
 		name = arg + 2;
-	} else if (prefix)
+	} else if (!no_wildcard(arg))
+		return 1;
+	else if (prefix)
 		name = prefix_filename(prefix, strlen(prefix), arg);
 	else
 		name = arg;