gitk: Avoid an error when cherry-picking if HEAD has moved on

This fixes an error reported by Adam Piątyszek: if the current HEAD
is not in the graph that gitk knows about when we do a cherry-pick
using gitk, then gitk hits an error when trying to update its
internal representation of the topology.  This avoids the error by
not doing that update if the HEAD before the cherry-pick was a
commit that gitk doesn't know about.
Signed-off-by: NPaul Mackerras <paulus@samba.org>

gitk

@@ -6648,7 +6648,7 @@ proc addnewchild {id p} {
     global arcnos arcids arctags arcout arcend arcstart archeads growing
     global seeds allcommits
 
-    if {![info exists allcommits]} return
+    if {![info exists allcommits] || ![info exists arcnos($p)]} return
     lappend allids $id
     set allparents($id) [list $p]
     set allchildren($id) {}