Allow for Oh My Zsh integration

Start replacing $ZSH variable with $DOTFILES in a backward-compatible
way, so that existing users have a chance to migrate.

Cherry-picked from holman#272.
Fixed conflicts.

git/gitconfig.symlink

@@ -5,9 +5,9 @@
         protocol = https
 [alias]
         co = checkout
-        promote = !$ZSH/bin/git-promote
-        wtf     = !$ZSH/bin/git-wtf
-        rank-contributors = !$ZSH/bin/git-rank-contributors
+        promote = !$DOTFILES/bin/git-promote
+        wtf     = !$DOTFILES/bin/git-wtf
+        rank-contributors = !$DOTFILES/bin/git-rank-contributors
         count   = !git shortlog -sn
 [color]
         diff = auto

zsh/prompt.zsh

@@ -62,7 +62,7 @@ battery_status() {
 
   if [[ $(sysctl -n hw.model) == *"Book"* ]]
   then
-    $ZSH/bin/battery-status
+    $DOTFILES/bin/battery-status
   fi
 }
 

zsh/zshrc.symlink

@@ -1,7 +1,10 @@
 # Fig pre block. Keep at the top of this file.
 [[ -f "$HOME/.fig/shell/zshrc.pre.zsh" ]] && . "$HOME/.fig/shell/zshrc.pre.zsh"
-# shortcut to this dotfiles path is $ZSH
-export ZSH=$HOME/.dotfiles
+# shortcut to this dotfiles path is $DOTFILES
+export DOTFILES=$HOME/.dotfiles
+
+# $ZSH variable is deprecated, use $DOTFILES instead
+export ZSH="$DOTFILES"
 
 # your project folder that we can `c [tab]` to
 export PROJECTS=~/Code
@@ -16,7 +19,7 @@ fi
 
 # all of our zsh files
 typeset -U config_files
-config_files=($ZSH/**/*.zsh)
+config_files=($DOTFILES/**/*.zsh)
 
 # load the path files
 for file in ${(M)config_files:#*/path.zsh}
@@ -30,9 +33,20 @@ do
   source $file
 done
 
-# initialize autocomplete here, otherwise functions won't be loaded
-autoload -U compinit
-compinit
+# Oh My Zsh could be installed somewhere out of dotfiles directory tree, e.g.
+# into  ~/.oh-my-zsh; the configuration file created by Oh My Zsh installer
+# should be moved as follows: {.zshrc => $DOTFILES/zsh/oh-my-zshrc}.
+if [[ -f $DOTFILES/zsh/oh-my-zshrc ]]; then
+  () {
+    local ZSH="$HOME/.oh-my-zsh"
+    source $DOTFILES/zsh/oh-my-zshrc
+    export OHMYZSH="$ZSH"
+  }
+else
+  # initialize autocomplete here, otherwise functions won't be loaded
+  autoload -U compinit
+  compinit
+fi
 
 # load every completion after autocomplete loads
 for file in ${(M)config_files:#*/completion.zsh}
@@ -51,5 +65,8 @@ zle -N down-line-or-beginning-search
 bindkey "^[[A" up-line-or-beginning-search # Up
 bindkey "^[[B" down-line-or-beginning-search # Down
 
+# Notify existing users that they should migrate to another variable.
+export ZSH='$ZSH is deprecated, use either $DOTFILES or $OHMYZSH instead.'
+
 # Fig post block. Keep at the bottom of this file.
 [[ -f "$HOME/.fig/shell/zshrc.post.zsh" ]] && . "$HOME/.fig/shell/zshrc.post.zsh"