Use a vcs library and add a Mercurial plugin

Move git_* functions to vcs_* to allow for better plugin support
Modify plugins to use the new function names and declare new variable
names, so ZSH_THEME_GIT_* variables are ZSH_THEME_VCS_* and
git_prompt_info() is vcs_prompt_info()

lib/vcs.zsh

@@ -0,0 +1,26 @@
+function vcs_name() {
+  for vcs in $VCS; do
+    if ${vcs}_check; then
+      echo $vcs
+      break
+    fi
+  done
+}
+
+# get the name of the branch we are on
+
+function vcs_prompt_info() {
+  local vcs=$(vcs_name)
+  [[ -n $vcs ]] && ${vcs}_prompt_info
+}
+
+function parse_vcs_dirty () {
+  local vcs=$(vcs_name)
+  [[ -n $vcs ]] && parse_${vcs}_dirty
+}
+
+# get the status of the working tree
+function vcs_prompt_status() {
+  local vcs=$(vcs_name)
+  [[ -n $vcs ]] && ${vcs}_prompt_status
+}