zsh_completion: declare appropriate local parameters for ->string form

When ->string form is used for _arguments, the function that calls it must
declare appropriate local parameters.

Managing local return value is needed to tell the completion system if our
function succeeded in suggesting something or not, plus without that `hg diff
-<TAB>` doesn't look right.

While at it, fix a copypaste error (s/diff_files/revert_files/).

Docs: http://zsh.sourceforge.net/Doc/Release/Completion-System.html

Differential Revision: https://phab.mercurial-scm.org/D4264

contrib/zsh_completion

@@ -581,7 +581,9 b' typeset -A _hg_cmd_globals'
 }
 
 _hg_cmd_diff() {
+  local context state state_descr line ret=1
   typeset -A opt_args
+
   _arguments -s -S : $_hg_global_opts $_hg_diff_opts $_hg_ignore_space_opts \
                      $_hg_pat_opts $_hg_subrepos_opts \
   '*'{-r+,--rev=}'[revision]:revision:_hg_revrange' \
@@ -591,17 +593,19 b' typeset -A _hg_cmd_globals'
   '--reverse[produce a diff that undoes the changes]' \
   '(--unified -U)'{-U+,--unified=}'[number of lines of context to show]:' \
   '--stat[output diffstat-style summary of changes]' \
-  '*:file:->diff_files'
+  '*:file:->diff_files' && ret=0
 
   if [[ $state == 'diff_files' ]]
   then
     if [[ -n $opt_args[-r] ]]
     then
-      _hg_files
+      _hg_files && ret=0
     else
-      _hg_committable
+      _hg_committable && ret=0
     fi
   fi
+
+  return ret
 }
 
 _hg_cmd_export() {
@@ -796,7 +800,7 b' typeset -A _hg_cmd_globals'
 }
 
 _hg_cmd_resolve() {
-  local context state line
+  local context state state_descr line ret=1
   typeset -A opt_args
 
   _arguments -s -S : $_hg_global_opts $_hg_mergetool_opts $_hg_pat_opts \
@@ -805,17 +809,19 b' typeset -A _hg_cmd_globals'
   '(--list -l --mark -m --unmark -u)'{-l,--list}'[list state of files needing merge]:*:merged files:->resolve_files' \
   '(--mark -m --list -l --unmark -u)'{-m,--mark}'[mark files as resolved]:*:unresolved files:_hg_unresolved' \
   '(--unmark -u --list -l --mark -m)'{-u,--unmark}'[unmark files as resolved]:*:resolved files:_hg_resolved' \
-  '*:file:_hg_unresolved'
+  '*:file:_hg_unresolved' && ret=0
 
   if [[ $state == 'resolve_files' ]]
   then
     _alternative 'files:resolved files:_hg_resolved' \
-      'files:unresolved files:_hg_unresolved'
+      'files:unresolved files:_hg_unresolved' && ret=0
   fi
+
+  return ret
 }
 
 _hg_cmd_revert() {
-  local context state line
+  local context state state_descr line ret=1
   typeset -A opt_args
 
   _arguments -s -S : $_hg_global_opts $_hg_pat_opts $_hg_dryrun_opts \
@@ -823,19 +829,21 b' typeset -A _hg_cmd_globals'
   '(--rev -r)'{-r+,--rev=}'[revision to revert to]:revision:_hg_labels' \
   '(--no-backup -C)'{-C,--no-backup}'[do not save backup copies of files]' \
   '(--date -d)'{-d+,--date=}'[tipmost revision matching date]:date code:' \
-  '*:file:->diff_files'
+  '*:file:->revert_files' && ret=0
 
-  if [[ $state == 'diff_files' ]]
+  if [[ $state == 'revert_files' ]]
   then
     if [[ -n $opt_args[-r] ]]
     then
-      _hg_files
+      _hg_files && ret=0
     else
       typeset -a status_files
       _hg_status mard
-      _wanted files expl 'modified, added, removed or deleted file' _multi_parts / status_files
+      _wanted files expl 'modified, added, removed or deleted file' _multi_parts / status_files && ret=0
     fi
   fi
+
+  return ret
 }
 
 _hg_cmd_rollback() {