grep: move nested func out to the class

This is in preparation for adding jobs support.  The nested function
is referenced in the options object which can't be pickled, so pull
it out into a static method instead.

Change-Id: I280ed2bf26390a0203925517a0d17c13053becaa
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/297983


Reviewed-by: Michael Mortensen <mmortensen@google.com>
Tested-by: Mike Frysinger <vapier@google.com>