gitk: Fix bug causing Tcl error when no commits are selected

Some of the stuff that commit 31c0eaa8cc
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>

gitk

@@ -4573,7 +4573,7 @@ proc drawvisible {} {
 
     set fs [$canv yview]
     set ymax [lindex [$canv cget -scrollregion] 3]
-    if {$ymax eq {} || $ymax == 0} return
+    if {$ymax eq {} || $ymax == 0 || $numcommits == 0} return
     set f0 [lindex $fs 0]
     set f1 [lindex $fs 1]
     set y0 [expr {int($f0 * $ymax)}]