`MergeRequest#source_sha` is expected to return the sha of the source
branch last commit.
But when a open Merge Request has no diff (e.g. all commits have already
been merged to the target branch), `merge_request.source_sha`
incorrectly returns `nil`.
This was un-noticed before – but now that !2217 has been merged,
it makes `Gitlab::Git::Commit.between` raise an
"Unexpected nil argument" exception.
This fixes the crash, by making sure that `source_sha` returns a
correct result even when there is no diff available.
18295585d9