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This makes it clearer and possibly more portable for different
platforms.

For now this supports:
- Mac OS X
- Linux (presumably works on all versions)
- Cygwin (Windows)

This prevents it from malfunctioning when `setopt prompt_subst` is off.

Partially fixes #3614

This adds two new theme functions for git:
* `git_prompt_behind` works in a identical fashion to `git_prompt_ahead`
  and will output a format variable (`ZSH_THEME_GIT_PROMPT_BEHIND`) if
  the branch is behind.
* `git_prompt_remote` will output one format variable if the branch
  exists on remote (`ZSH_THEME_GIT_PROMPT_REMOTE_EXISTS`), and another
  if the branch is unpublished (`ZSH_THEME_GIT_PROMPT_REMOTE_MISSING`).

The old `git_prompt_ahead` has been changed. Using git log is subject
to formatting in .gitconfig, which can be overridden and will break
this function. Relying on rev-list is much more stable.

Do not put explicit dot in $cdpath, since it causes problems on Cygwin due to an underlying zsh bug.

The -h options in l, ll, and la were lost in 25b1cd68. There's no reason
to remove them.

LANG is a colon-separated list of prefered locales; LC_CTYPE is single entry,
therefore we need to remove all but one entry.

Also, there is no need setting it if it is already set (LC_ALL also sets
LC_CTYPE, so we check it too).

Move the DISABLE_AUTO_TITLE check from title() to the preX hook
functions that call it, to allow the title() function to be used
directly by user or other callers.

There were only three aliases left in aliases.zsh
File deleted!

More compact case statement instead of the ugly if elif... else
statement. Moves it to history.zsh

- Cleans out aliases.zsh.
- Removes unneeded cd function.

contaminating script/function output

We are running oh-my-zsh on a shared install and the only issue is each
user having a custom cache dir. This will allow us to set the cache dir
to ZSH_CACHE_DIR=~/oh-my-zsh/cache/ .

I've seen stranger things on OS X, unfortunately.
This reverts commit 1f5cecee4768be192e439a72a873a6cfe8720927.

Revert PR #3359 as this is breaking stuff for many folks. Need to rethink how we handle system defaults differently on this