Offering auto completion for existing commits is great, but on big
repositories, it suddenly becomes really slow, even with fast hard
disks, since each commit is read and then a line processed for it.
Instead limit to the last 500 commits (arbitrary number) which still
feels fast. Going back further in history can easily and more reasonably
done with git log etc.
This can be prohibitively slow on large repositories (minutes!).
While regrettable, no user is going to like waiting that long.
Work towards #3342, rerun of #3230.
Many thanks to @gladhorn for the idea!