[hg] Allow turning off the hg status call in a similar fashion to the git plugin

In some very large repositories, `hg status` can take a pretty long time. It doesn't feel great to have a multi second delay every time you run a command while you wait for hg status to complete.

Mirroring the functionality in the git plugin, this diff adds a check for a new configuration option in `hg config`. If this option is enabled we disable the `hg status` check and just show the clean marker.

plugins/mercurial/mercurial.plugin.zsh

@@ -20,7 +20,7 @@ alias hgca='hg qimport -r tip ; hg qrefresh -e ; hg qfinish tip'
 alias hgun='hg resolve --list'
 
 function in_hg() {
-  if [[ -d .hg ]] || $(hg summary > /dev/null 2>&1); then
+  if [[ -d .hg ]] || $(hg root > /dev/null 2>&1); then
     echo 1
   fi
 }
@@ -42,6 +42,12 @@ $ZSH_THEME_REPO_NAME_COLOR$_DISPLAY$ZSH_PROMPT_BASE_COLOR$ZSH_PROMPT_BASE_COLOR$
 
 function hg_dirty_choose {
   if [ $(in_hg) ]; then
+    # Allow turning off the 'hg status' call. Useful in large repos where that
+    # can take a while. Mirror the functionality of the git plugin by just
+    # showing the "clean" prompt in this case.
+    if [[ "$(command hg config oh-my-zsh.hide-dirty)" = "1" ]]; then
+      echo $2
+    else
       hg status 2> /dev/null | command grep -Eq '^\s*[ACDIM!?L]'
       if [ $pipestatus[-1] -eq 0 ]; then
         # Grep exits with 0 when "One or more lines were selected", return "dirty".
@@ -51,6 +57,7 @@ function hg_dirty_choose {
         echo $2
       fi
     fi
+  fi
 }
 
 function hg_dirty {