fix(single_merge_job): add git timeout comments

If a git command times out while marge is handling an MR, marge leaves an "Unexpected exception" comment on the MR and terminates.

Instead, add a more detailed comment on the MR that a git command has timed out and don't terminate the marge process. This gives the developer a hint that immediately reassigning the MR might succeed or that the git-timeout configuration needs to be adjusted.

Motived by this issue: https://gitlab.freedesktop.org/freedesktop/marge-bot/-/issues/4