Support cvs via git-shell

Support cvs via git-shell

This adds cvs support to the git-shell; You can now give new users
a restricted git-shell and they still can commit via git's cvs
emulator.

Note that either the gecos information must be accurate, or you must
provide a $HOME/.gitconfig with the appropriate user credentials.
Since the git-shell is too restricted to allow the user to do it
(on purpose!), it is up to the administrator to take care of that.

Based on an idea by Jan Wielemaker.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Lars Hjemli <hjemli@gmail.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>