gitk: Fix bug causing nearby tags/heads to sometimes not be displayed

When we compute descendent heads and descendent/ancestor tags, we
cache the results.  We need to be careful to invalidate the cache
when we add stuff to the graph.  Also make sure that when we cache
descendent heads for a node we only cache the heads that are actually
descendents of that node.
Signed-off-by: NPaul Mackerras <paulus@samba.org>

gitk

@@ -5950,7 +5950,7 @@ proc regetallcommits {} {
 # coming from descendents, and "outgoing" means going towards ancestors.
 
 proc getallclines {fd} {
-    global allids allparents allchildren idtags nextarc nbmp
+    global allids allparents allchildren idtags idheads nextarc nbmp
     global arcnos arcids arctags arcout arcend arcstart archeads growing
     global seeds allcommits
 
@@ -6023,6 +6023,12 @@ proc getallclines {fd} {
 	}
 	set arcout($id) $ao
     }
+    if {$nid > 0} {
+	global cached_dheads cached_dtags cached_atags
+	catch {unset cached_dheads}
+	catch {unset cached_dtags}
+	catch {unset cached_atags}
+    }
     if {![eof $fd]} {
 	return [expr {$nid >= 1000? 2: 1}]
     }
@@ -6674,7 +6680,7 @@ proc descheads {id} {
     if {![info exists allparents($id)]} {
 	return {}
     }
-    set ret {}
+    set aret {}
     if {[llength $arcnos($id)] == 1 && [llength $allparents($id)] == 1} {
 	# part-way along an arc; check it first
 	set a [lindex $arcnos($id) 0]
@@ -6684,7 +6690,7 @@ proc descheads {id} {
 	    foreach t $archeads($a) {
 		set j [lsearch -exact $arcids($a) $t]
 		if {$j > $i} break
-		lappend $ret $t
+		lappend aret $t
 	    }
 	}
 	set id $arcstart($a)
@@ -6692,6 +6698,7 @@ proc descheads {id} {
     set origid $id
     set todo [list $id]
     set seen($id) 1
+    set ret {}
     for {set i 0} {$i < [llength $todo]} {incr i} {
 	set id [lindex $todo $i]
 	if {[info exists cached_dheads($id)]} {
@@ -6714,6 +6721,7 @@ proc descheads {id} {
     }
     set ret [lsort -unique $ret]
     set cached_dheads($origid) $ret
+    return [concat $ret $aret]
 }
 
 proc addedtag {id} {