Add git-stash script

Add git-stash script

When my boss has something to show me and I have to update, for some
reason I am always in the middle of doing something else, and git pull
command refuses to work in such a case.

I wrote this little script to save the changes I made, perform the
update, and then come back to where I was, but on top of the updated
commit.

This is how you would use the script:

  $ git stash
  $ git pull
  $ git stash apply

[jc: with a few fixlets from the list]

Signed-off-by: Nanako Shiraishi <nanako3@bluebottle.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>