Teach for-each-ref about a little language called Tcl.

Teach for-each-ref about a little language called Tcl.

Love it or hate it, some people actually still program in Tcl.  Some
of those programs are meant for interfacing with Git.  Programs such as
gitk and git-gui.  It may be useful to have Tcl-safe output available
from for-each-ref, just like shell, Perl and Python already enjoy.

Thanks to Sergey Vlasov for pointing out the horrible flaws in the
first and second version of this patch, and steering me in the right
direction for Tcl value quoting.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>