Add a simple script to create a working directory that uses symlinks
to point at an exisiting repository.  This allows having different
branches in different working directories but all from the same
repository.

Based on a description from Junio of how he creates multiple working
directories[1].  With the following caveat:

"This risks confusion for an uninitiated if you update a ref that
is checked out in another working tree, but modulo that caveat
it works reasonably well."

[1] http://article.gmane.org/gmane.comp.version-control.git/41513/Signed-off-by: NJulian Phillips <julian@quantumfyre.co.uk>
Signed-off-by: NJunio C Hamano <junkio@cox.net>