added alias: git undo (#123)

Credits to [Enrico Campidoglio](http://megakemp.com/about/),
see [original article](http://megakemp.com/2016/08/25/git-undo/).

Author: fibo

README.md

@@ -138,6 +138,7 @@ P.S: All these commands are tested on `git version 2.7.4 (Apple Git-66)`.
 * [Show how many lines does an author contribute](#show-how-many-lines-does-an-author-contribute)
 * [Revert: Reverting an entire merge](#revert-reverting-an-entire-merge)
 * [Number of commits in a branch](#number-of-commits-in-a-branch)
+* [Alias: git undo](#alias-git-undo)
 
 <!-- Don’t remove or change the comment below – that can break automatic updates. More info at <http://npm.im/doxie.inject>. -->
 <!-- @doxie.inject end toc -->
@@ -942,5 +943,10 @@ git revert -m 1 <commit-ish>
 git rev-list --count <branch-name>
 ```
 
+## Alias: git undo
+```sh
+git config --global alias.undo '!f() { git reset --hard $(git rev-parse --abbrev-ref HEAD)@{${1-1}}; }; f'
+```
+
 <!-- Don’t remove or change the comment below – that can break automatic updates. More info at <http://npm.im/doxie.inject>. -->
 <!-- @doxie.inject end -->

tips.json

@@ -403,4 +403,7 @@
 },{
     "title": "Number of commits in a branch",
     "tip": "git rev-list --count <branch-name>"
+},{
+    "title": "Alias: git undo",
+    "tip": "git config --global alias.undo '!f() { git reset --hard $(git rev-parse --abbrev-ref HEAD)@{${1-1}}; }; f'"
 }]