setup.c:verify_non_filename(): don't die unnecessarily while disambiguating

setup.c:verify_non_filename(): don't die unnecessarily while disambiguating

If you have a working tree _file_ "foo", attempt to refer to a
branch "foo/bar" without -- to disambiguate, like this:

	$ git log foo/bar

tried to make sure that foo/bar cannot be naming a working tree
file "foo/bar" (in which case we would say "which one do you
want?  A rev or a working tree file?  clarify with -- please").
We run lstat("foo/bar") to check that.  If it does not succeed,
there is no ambiguity.

That is good.  But we also checked the error status for the
lstat() and expected it to fail with ENOENT.  In this particular
case, however, it fails with ENOTDIR.  That should be treated as
"expected error" as well.

Signed-off-by: Junio C Hamano <gitster@pobox.com>