[Funky] "git -p cmd" inside a bare repository

If you have a bare repository and try this there:

	$ PAGER=head git show HEAD:gcc/ChangeLog

it works as expected, but if you explicitly ask for pagination, like this:

	$ PAGER=head git -p show HEAD:gcc/ChangeLog

you would get a very funky error message:

	fatal: ambiguous argument 'HEAD:gcc/ChangeLog': unknown revision or path not in the working tree.
	Use '--' to separate paths from revisions

as if we are working with a repository with working tree.

I originally noticed this with ls-tree.  The symptom is a bit different:

	$ git -p ls-tree HEAD
	fatal: Not a valid object name HEAD

I _think_ what is happening is that setup_pager() tries to run git_config(), which runs setup(), and then RUN_SETUP set for "ls-tree" (or "show") internal command runs setup again. HEAD is given to resolve_ref() and git_path("%s", ref) makes it to ".git/HEAD", even though in a bare repository git_dir should be set to ".", and of course we cannot find such a path in the git directory.