gitk: Fix bug in fix for warning when removing a branch

My fix in commit b1054ac985 was only
half-right, since it ignored the case where the descendent heads of
the head being removed correspond to two or more different commits.
This fixes it.  Reported by Mark Levedahl.

Signed-off-by: Paul Mackerras <paulus@samba.org>

gitk

@@ -6208,7 +6208,7 @@ proc rmbranch {} {
 	return
     }
     set dheads [descheads $id]
-    if {$idheads($dheads) eq $head} {
+    if {[llength $dheads] == 1 && $idheads($dheads) eq $head} {
 	# the stuff on this branch isn't on any other branch
 	if {![confirm_popup "The commits on branch $head aren't on any other\
 			branch.\nReally delete branch $head?"]} return