determine .git directory regardless of position

[alfredodeza]

Otherwise it forces one to be in the top level dir:

(ceph-volume) papaya-2[ceph-3.2-rhel-patches] ..pstream/ceph/src/ceph-volume ᓆ downstream-cherry-picker https://github.com/ceph/ceph/pull/24251 1640086
WARNING: .git/hooks/prepare-commit-msg not found, downloading from Gist
Traceback (most recent call last):
  File "/Users/alfredo/.virtualenvs/ceph-volume/bin/downstream-cherry-picker", line 6, in <module>
    exec(compile(open(__file__).read(), __file__, 'exec'))
  File "/Users/alfredo/python/downstream-cherry-picker/bin/downstream-cherry-picker", line 7, in <module>
    main(sys.argv)
  File "/Users/alfredo/python/downstream-cherry-picker/downstream_cherry_picker/__init__.py", line 161, in main
    ensure_hook()
  File "/Users/alfredo/python/downstream-cherry-picker/downstream_cherry_picker/__init__.py", line 133, in ensure_hook
    fh = open(hook, 'w')
IOError: [Errno 2] No such file or directory: '.git/hooks/prepare-commit-msg'

[alfredodeza]

I was just hit by this again :( I'll try to find a solution