[UPDATE] Plugin: git

## Update branch

**Plugin:** `git`

This branch contains updates for the `git` plugin. If you want to add a commit, please [submit a PR](https://github.com/ncanceill/oh-my-zsh/pull/new/plugin-git) to [my branch](https://github.com/ncanceill/oh-my-zsh/tree/plugin-git).

## Status

**READY FOR MERGE** (but testing cannot hurt)

~~I am still looking through all the open issues, and will add more stuff concerning the `git` plugin. After that we should allow some time for testing.~~ I think we got most of the related PRs, but even though the changes are pretty simple up to now, _it is always good to test_ (as GLaDOS would surely agree).

@Stibbons maybe you wish to help or to manage the branch yourself? @robbyrussell is that what you meant in #2771 about merging _"a collection"_ of PRs? — or _"one PR to rule them all"_?

Also, I think the auto-closing feature [should kick in](#issuecomment-43131152) when this is merged, which will ease maintenance.

## Changelog

* Fixes #1500
* Fixes #1504
* Fixes #1546
* Fixes #1601
* Fixes #1800
* Fixes #1966
* Fixes #1982
* Fixes #2124
* Fixes #2338
* Fixes #2407
* Fixes #2433
* Fixes #2463
* Fixes #2493
* Uses #2494, fixes #1846
* Fixes #2546
* Fixes #2684
* Fixes #2714
* Fixes #2921
I am using `alias gfa="git fetch --all --prune"`.