[BUG] :(attr) pathspecs can die("BUG") in the tree-walker

I was playing with the new :(attr) pathspecs in the upcoming v2.13 today, and noticed:

  $ git ls-files -- ':(attr:-diff)'
  t/t0110/url-1
  t/t0110/url-10
  [etc]

So far so good.

  $ git ls-tree HEAD -- ':(attr:-diff)'
  fatal: :(attr:-diff): pathspec magic not supported by this command: 'attr'

Bummer, but I understand that sometimes the options need to be plumbed through to work everywhere.

  $ git log HEAD -- ':(attr:-diff)'
  fatal: BUG:tree-walk.c:947: unsupported magic 40

Whoops. This is presumably ls-tree is protected, but I think we are missing a GUARD_PATHSPEC call somewhere.

This isn't a huge deal, as the correct behavior is probably to die like ls-tree does, but we probably shouldn't be hitting BUG assertions as a general rule.

-Peff