grep: use get_pathspec() correctly
grep: use get_pathspec() correctly

When there is no remaining string in argv, get_pathspec(prefix, argv)
will return a two-element array that has prefix as the first element,
so there is no need to re-roll that logic in the code that uses
get_pathspec().

Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git