copies-rust: make the comparison aware of the revision being current merged
This make no significant performance change in practice (all ±2%) in practice,
but it will help us to distinct between some semantically different cases later
on.
Differential Revision: https://phab.mercurial-scm.org/D9498
$ cat >> $HGRCPATH << EOF
> [command-templates]
> log="{rev}:{node|short} ({phase}) [{tags} {bookmarks}] {desc|firstline}\n"
> [extensions]
> dirstateparanoidcheck = $TESTDIR/../contrib/dirstatenonnormalcheck.py
> [experimental]
> nonnormalparanoidcheck = True
> [devel]
> all-warnings=True
> EOF
$ mkcommit() {
> echo "$1" > "$1"
> hg add "$1"
> hg ci -m "add $1"
> }
$ hg init testrepo
$ cd testrepo
$ mkcommit a
$ mkcommit b
$ mkcommit c
$ hg status