gitk: Fix bug causing undefined variable error when cherry-picking

When "Show nearby tags" is turned off and the user did a cherry-pick,
we were trying to access variables relating to the descendent/ancestor
tag & head computations in addnewchild though they hadn't been set.
This makes sure we don't do that.  Reported by Johannes Sixt.

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

gitk

@@ -6648,8 +6648,9 @@ proc splitarc {p} {
 proc addnewchild {id p} {
     global allids allparents allchildren idtags nextarc nbmp
     global arcnos arcids arctags arcout arcend arcstart archeads growing
-    global seeds
+    global seeds allcommits
 
+    if {![info exists allcommits]} return
     lappend allids $id
     set allparents($id) [list $p]
     set allchildren($id) {}