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This patch makes git_prompt_status support three new status variables:

 - ZSH_THEME_GIT_PROMPT_AHEAD
 - ZSH_THEME_GIT_PROMPT_BEHIND
 - ZSH_THEME_GIT_PROMPT_DIVERGED

With these extra variables it's easy to see (1) if you have commits in
your local branch that weren't pushed to the remote (AHEAD), (2) if
there are commits in the remote that you haven't merged/rebased yet
(BEHIND) or (3) if you have local unpushed commits AND the remote has
some commits you haven't merged yet (DIVERGED).

Refer to the first line displayed on `git status -b --porcelain`.

An example setup in a .zsh-theme file would be:

ZSH_THEME_GIT_PROMPT_AHEAD="↑"
ZSH_THEME_GIT_PROMPT_BEHIND="↓"
ZSH_THEME_GIT_PROMPT_DIVERGED="↕"

Esc+. works as "last arg" on both bash and zsh. Seems like a shame to introduce a new standard.

Main library should use ZSH native hostname function

This is needed to play nice with plugins that need various hooks.

After a few weeks of using this, I'd prefer this to not be enabled by default. My bad for suggesting that in the first place.

This reverts commit fc49b4a4.

The command C-x C-e opens the current command line for editing
in the editor defined by the $EDITOR environment variable.

Thanks to Craig Bosma and his blog post about this:
http://distrustsimplicity.net/articles/zsh-command-editing

Changes method git_parse_ahead() on git.zsh lib file. Before this
change it checked directly against origin/master branch. Now it
uses $(current_branch) to check for not pushed changes against
the current remote branch.

Modifies the Git lib file (lib/git.zsh), adding three new prompt
methods:

- git_prompt_ahead(): Shows the content of the custom var
  $ZSH_THEME_GIT_PROMPT_AHEAD if the local repository has
  commits ahead from the remote origin repository

- git_prompt_short_sha(): Shows last commit SHA hash in short
  mode wrapped between the content of the custom vars
  $ZSH_THEME_GIT_PROMPT_SHA_BEFORE and
  $ZSH_THEME_GIT_PROMPT_SHA_AFTER

- git_prompt_long_sha(): Shows last commit SHA hash in long
  mode wrapped between the content of the custom vars
  $ZSH_THEME_GIT_PROMPT_SHA_BEFORE and
  $ZSH_THEME_GIT_PROMPT_SHA_AFTER

This reverts commit c3a58b00.

Fix by derekjw. closes #52.

Closes #45 #57 #58 #86.