gitk: Fix warning when removing a branch

When we had two heads on the same commit, and the user tried to remove
one of them, gitk was sometimes incorrectly saying that the commits
on that branch weren't on any other branch.  This fixes it.

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

gitk

@@ -6168,7 +6168,7 @@ proc cobranch {} {
 
 proc rmbranch {} {
     global headmenuid headmenuhead mainhead
-    global headids idheads
+    global idheads
 
     set head $headmenuhead
     set id $headmenuid
@@ -6178,7 +6178,7 @@ proc rmbranch {} {
 	return
     }
     set dheads [descheads $id]
-    if {$dheads eq $headids($head)} {
+    if {$idheads($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