Show branch id if not on tip

If not on the tip it's often useful to know where we are exactly. This
commit brings this feature. If on the tip, then just the branch name is
displayed, but if not, then it's displayed like that: `branchname@id`.

plugins/mercurial/mercurial.plugin.zsh

@@ -38,13 +38,23 @@ function in_hg() {
   fi
 }
 
+function hg_get_branch_id() {
+  local hg_id_tip=`hg log -T "{node|short}" -l 1 -b .`
+  local hg_id=`hg id -i -r .`
+
+  if [[ $hg_id != $hg_id_tip ]]; then
+    echo "@${hg_id}"
+  fi
+}
+
 function hg_get_branch_name() {
   local hg_dir=$(hg_get_dir)
   if [[ $hg_dir != "" ]]; then
+    local hg_id=$(hg_get_branch_id)
     if [[ -f "${hg_dir}/branch" ]]; then
-      echo $(<"${hg_dir}/branch")
+      echo $(<"${hg_dir}/branch")$hg_id
     else
-      echo "default"
+      echo "default${hg_id}"
     fi
   fi
 }