The CommitsBetween RPC call has been deprecated by Gitaly in favor of
ListCommits, which is a much more flexible alternative which allows the
caller to specify a set of revisions and various other parameters.
Reimplement `#between` on top of `#list_commits` such that Gitaly can
eventually drop CommitsBetween.

Note that instead of using the range-notation "fromrev..torev", we use
its equivalent spelling "^fromrev torev". See also the gitrevisions(7)
man page.