fix OSError on “git fat pull” with manually removed, tracked files in WC

If there is any manually removed, tracked file in the working copy, then
running “git fat pull” fails with an OSError without this fix.
Merge branch 'fix-OSError-on-removed-tracked-file' of github:cspurk/git-fat (PR #30)

* 'fix-OSError-on-removed-tracked-file' of github:cspurk/git-fat:
  fix OSError on “git fat pull” with manually removed, tracked files in WC