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

contrib/zsh_completion

@@ -624,7 +624,7 b' typeset -A _hg_cmd_globals'
 
   if [[ $state == 'diff_files' ]]
   then
-    if [[ -n $opt_args[-r] ]]
+    if [[ -n ${opt_args[(I)-r|--rev]} ]]
     then
       _hg_files && ret=0
     else
@@ -870,7 +870,7 b' typeset -A _hg_cmd_globals'
 
   if [[ $state == 'revert_files' ]]
   then
-    if [[ -n $opt_args[-r] ]]
+    if [[ -n ${opt_args[(I)-r|--rev]} ]]
     then
       _hg_files && ret=0
     else