Add Clean Up Old Remote Tracking References as a git til.

README.md

@@ -56,6 +56,7 @@ smart people at [Hashrocket](http://hashrocket.com/).
 - [Checkout Old Version Of A File](git/checkout-old-version-of-a-file.md)
 - [Checkout Previous Branch](git/checkout-previous-branch.md)
 - [Clean Out All Local Branches](git/clean-out-all-local-branches.md)
+- [Clean Up Old Remote Tracking References](git/clean-up-old-remote-tracking-references.md)
 - [Delete All Untracked Files](git/delete-all-untracked-files.md)
 - [Determine The Hash Id For A Blob](git/determine-the-hash-id-for-a-blob.md)
 - [Dry Runs in Git](git/dry-runs-in-git.md)

git/clean-up-old-remote-tracking-references.md

@@ -0,0 +1,17 @@
+# Clean Up Old Remote Tracking References
+
+After working on a Git-versioned project for a while, you may find that
+there are a bunch of references to remote branches in your local repository.
+You know those branches definitely don't exist on the remote server and
+you've removed the local branches, but
+you still have references to them lying around. You can reconcile this
+discrepancy with one command:
+
+```bash
+$ git fetch origin --prune
+```
+
+This will prune all those non-existent remote tracking references which is
+sure to clean up your git log (`git log --graph`).
+
+[source](http://stackoverflow.com/a/3184742/535590)