git-gui: Don't require $DISPLAY just to get --version

git-gui: Don't require $DISPLAY just to get --version

Junio asked that we don't force the user to have a valid X11 server
configured in $DISPLAY just to obtain the output of `git gui version`.
This makes sense, the user may be an automated tool that is running
without an X server available to it, such as a build script or other
sort of package management system.  Or it might just be a user working
in a non-GUI environment and wondering "what version of git-gui do I
have installed?".

Tcl has a lot of warts, but one of its better ones is that a comment
can be continued to the next line by escaping the LF that would have
ended the comment using a backslash-LF sequence.  In the past we have
used this trick to escape away the 'exec wish' that is actually a Bourne
shell script and keep Tcl from executing it.

I'm using that feature here to comment out the Bourne shell script and
hide it from the Tcl engine.  Except now our Bourne shell script is a
few lines long and checks to see if it should print the version, or not.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>