First draft on Git tricks tutorial

[ci skip]

doc/tutorials/git_tricks.md

+Based on https://gitlab.com/gitlab-org/gitlab-ce/issues/5986 here is an outline.
+
+---
+
+A pack of Git tricks that will leverage your Git-fu.
+
+## Introduction
+
+
+## Oh-my-zsh Git plugin
+
+- https://github.com/robbyrussell/oh-my-zsh/wiki/Plugin:git
+- https://github.com/robbyrussell/oh-my-zsh/blob/master/plugins/git/git.plugin.zsh
+
+## Git extras
+
+Enhance Git with more commands
+
+- https://github.com/tj/git-extras
+
+## Aliases
+
+```ini
+[alias]
+  lg = log --graph --pretty=format:'%Cred%h%Creset -%C(yellow)%d%Creset %s %Cgreen(%cr)%Creset' --abbrev-commit --date=relative
+  lol = log --graph --decorate --pretty=oneline --abbrev-commit
+```
+
+## `.gitconfig` on steroids
+
+- https://github.com/thoughtbot/dotfiles/blob/master/gitconfig
+- https://github.com/thoughtbot/dotfiles/pull/377
+
+---
+
+1.  Set a global `.gitignore`:
+
+    ```ini
+    [core]
+      excludesfile = /home/user/.gitignore
+    ```
+
+1.  Delete local branches that have been removed from remote on fetch/pull:
+
+    ```ini
+    [fetch]
+      prune = true
+    ```
+
+1.  Gives you extra info when using Git submodules:
+
+    ```ini
+    [status]
+      submodulesummary = 1
+    ```
+
+## Misc
+
+1. Get a list of Git branches, ordered by most recent commit:
+
+   ```
+   git for-each-ref --sort=-committerdate refs/heads/
+   ```
+
+1. `@` is the same as `HEAD`:
+
+    ```
+    git show @~3
+    ```
+
+1. `-` refers to the branch you were on before the current one.
+   Use it to checkout the previous branch ([source][dash]):
+
+    ```sh
+    % git branch
+      master
+    * rs-zenmode-refactor
+
+    % git checkout master
+
+    % git checkout -
+    ```
+
+1. Delete local branches which have already been merged into master
+   ([source][del-merged]):
+
+    ```
+    git branch --merged master | grep -v "master" | xargs -n 1 git branch -d
+    ```
+
+1.  Delete all stale tracking branches for a remote:
+
+    ```
+    git remote prune origin
+    ```
+
+[del-merged]: http://stevenharman.net/git-clean-delete-already-merged-branches
+[dash]: https://twitter.com/holman/status/530490167522779137