fix(single_merge_job): add git timeout comments

If a git command times out while marge is handling a single merge
request, marge will leave an "Unexpected exception" comment on the merge
request and then terminate.

Instead, catch the `TimeoutExpired` exception raised by the failed git
command and add a more detailed comment on the merge request about what
occured. Also, don't stop the marge process since the problem probably
did not come from marge herself.

This more detailed error message should alert developers to increase the
git timeout configuration if necessary.