phlgit_checkout: 'clean fxd' in orphan_clean()

Summary:
Cover the case where we're doing a clean orphan checkout and some of
the files in the index are also affected by the .gitignore

In this case we must also remove ignored files with the 'x' flag.

Test Plan:
$ ./precommit.sh
$ ./testbed/arcyd/acceptance_tests.sh

do this before and after fix to verify we're exercising the issue
$ ./testbed/arcyd/test_shell.sh
    $ touch BLAH
    $ git add BLAH
    $ echo BLAH >> .gitignore
    $ git add .
    $ git checkout r/master/BLAH
    $ git push -u origin r/master/BLAH
    $ ./accept.sh
    $ git checkout master
    $ ./poke.sh
    $ ./accept.sh

Reviewers: cbailey

Reviewed By: cbailey

CC: arcyd

Differential Revision: https://all.phab.dev.bloomberg.com/D47219