pre-commit: Add 'unset GREP_OPTIONS' (similar to Gerrit commit-msg)

  msysgit grep is fairly old and doesn't recognize certain
  grep options, e.g., '--color=auto'.

  I imagine there may possibly be other grep option issues
  on various platforms.

  (I also note that the various git scripts in libexec/git-core
   clear out GREP_OPTIONS before calling grep).

Change-Id: I67bc148a77cfc0167064e61e8c47a5f091704eac
Reviewed-on: https://code.wireshark.org/review/2945
Reviewed-by: Bill Meier <wmeier@newsguy.com>