Update git branches on push.

This pushes to clean checked-out repositories.  Only works since git
2.3.0, so on older versions, I'll apparently need to use a local dotfile
for those systems.

git config receive.denyCurrentBranch updateInstead

dotfiles/gitconfig

@@ -56,3 +56,5 @@
 
 [credential]
 	helper = cache --timeout=36000
+[receive]
+	denyCurrentBranch = updateInstead