zsh_completion: handle --rev as well as -r for diff and revert

Authored by av6.

Description

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