Some of the stuff that commit
31c0eaa8cc10944ebca8aa31fb59f0b77b1b6e77
added to drawvisible isn't appropriate to do when we have no commits,
and this was causing a Tcl error if gitk was invoked in such a fashion
that no commits were selected. This fixes it by bailing out of
drawvisible early if there are no commits displayed.
Bug reported by Johannes Sixt.
Signed-off-by: Paul Mackerras <paulus@samba.org>