Use SIGTERM on the process group when aborting

After reading the curious Git repository corruption report in
https://gitlab.com/gitlab-org/gitlab-ce/issues/2336 (where
gitlab-git-http-server was NOT being used, for the record), I feel
inclined to be a little more cautious and use SIGTERM instead of
SIGKILL to terminate the git-xxx-pack process group in case of
aborted requests.

Also discussed in
https://gitlab.com/gitlab-org/gitlab-git-http-server/issues/1 .