add: do not accept pathspec magic 'attr'

Commit b0db704652 (pathspec: allow querying for attributes -
2017-03-13) adds new pathspec magic 'attr' but only with
match_pathspec(). "git add" has some pathspec related code that still
does not know about 'attr' and will bail out:

    $ git add ':(attr:foo)'
    fatal: BUG:dir.c:1584: unsupported magic 40

A better solution would be making this code support 'attr'. But I
don't know how much work is needed (I'm not familiar with this new
magic). For now, let's simply reject this magic with a friendlier
message:

    $ git add ':(attr:foo)'
    fatal: :(attr:foo): pathspec magic not supported by this command: 'attr'

Update t6135 so that the expected error message is from the
"graceful" rejection codepath, not "oops, we were supposed to reject
the request to trigger this magic" codepath.

Reported-by: smaudet@sebastianaudet.com
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/add.c

@@ -433,7 +433,7 @@ int cmd_add(int argc, const char **argv, const char *prefix)
 	 * Check the "pathspec '%s' did not match any files" block
 	 * below before enabling new magic.
 	 */
-	parse_pathspec(&pathspec, 0,
+	parse_pathspec(&pathspec, PATHSPEC_ATTR,
 		       PATHSPEC_PREFER_FULL |
 		       PATHSPEC_SYMLINK_LEADING_PATH,
 		       prefix, argv);

t/t6135-pathspec-with-attrs.sh

@@ -166,7 +166,7 @@ test_expect_success 'fail if attr magic is used places not implemented' '
 	# though, but git-add is convenient as it has its own internal pathspec
 	# parsing.
 	test_must_fail git add ":(attr:labelB)" 2>actual &&
-	test_i18ngrep "unsupported magic" actual
+	test_i18ngrep "magic not supported" actual
 '
 
 test_expect_success 'abort on giving invalid label on the command line' '