Merge branch 'dm/unbash-subtree'

It turns out that git-subtree script does not have to be run with
bash.

* dm/unbash-subtree:
  contrib/git-subtree: Use /bin/sh interpreter instead of /bin/bash

contrib/subtree/git-subtree.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/bin/sh
 #
 # git-subtree.sh: split/join git repositories in subdirectories of this one
 #