git-gui: properly popup error if gitk should be started but is not installed

git-gui: properly popup error if gitk should be started but is not installed

On 'Visualize ...', a gitk process is started.  Since it is run in the
background, catching a possible startup error doesn't work, and the error
output goes to the console git-gui is started from.  The most probable
startup error is that gitk is not installed; so before trying to start,
check for the existence of the gitk program, and popup an error message
unless it's found.

This was noticed and reported by Paul Wise through
 http://bugs.debian.org/429810

Signed-off-by: Gerrit Pape <pape@smarden.org>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>