gitc_utils: rewrite to use multiprocessing

This is the only code in the tree that uses GitCommand asynchronously.
Rewrite it to use multiprocessing.Pool as it makes the code a little
bit easier to understand and simpler.

Change-Id: I3ed3b037f24aa1e9dfe8eec9ec21815cdda7678a
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/297143


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