RepositoryService.WriteRef: honour `old_revision`

This is a follow-up to #48 (closed)

The old_revision field of WriteRefRequest goes straight to the <oldvalue> argument of git-update-ref(1):

NAME
       git-update-ref - Update the object name stored in a ref safely

SYNOPSIS
       git update-ref [-m <reason>] [--no-deref] (-d <ref> [<oldvalue>] | [--create-reflog] <ref> <newvalue> [<oldvalue>] | --stdin [-z])

       (...)
       Given three arguments, stores the <newvalue> in the <ref>, possibly dereferencing the symbolic refs, after verifying that
       the current value of the <ref> matches <oldvalue>. E.g. git update-ref refs/heads/master <newvalue> <oldvalue> updates
       the master branch head to <newvalue> only if its current value is <oldvalue>. You can specify 40 "0" or an empty string
       as <oldvalue> to make sure that the ref you are creating does not exist.

At the %HGitaly1: Hg SHAs level, the Rails app calls WriteRef both for Gitaly (auxiliary Git repo) and HGitaly (the native Mercurial repo). Hence it is enough to perform the check in the Git repository only. This won't be enough for %HGitaly2: fully native

There's also a force boolean field to investigate.