zsh_completion: handle --rev as well as -r for diff and revert
Completion for hg diff and revert should suggest all files in context of a
non-current revision. The script used to look only for `-r foo`, and now it
also understands `--rev foo`.
Differential Revision: https://phab.mercurial-scm.org/D4427
Test issue2761
$ hg init
$ touch to-be-deleted
$ hg add
adding to-be-deleted
$ hg ci -m first
$ echo a > to-be-deleted
$ hg ci -m second
$ rm to-be-deleted
$ hg diff -r 0
Same issue, different code path
$ hg up -C
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ touch does-not-exist-in-1
$ hg add
adding does-not-exist-in-1
$ hg ci -m third
$ rm does-not-exist-in-1
$ hg diff -r 1