gitk: Fix file list context menu for merge commits

Currently it displays an ugly error box, because the treediffs array
is not filled for such commits.  This fixes it by making
getmergediffline add the filenames it sees to the treediffs array
like gettreediffline does.
Signed-off-by: NAlexander Gavrilov <angavrilov@gmail.com>
Signed-off-by: NPaul Mackerras <paulus@samba.org>

gitk

@@ -6418,7 +6418,7 @@ proc getblobline {bf id} {
 
 proc mergediff {id} {
     global diffmergeid mdifffd
-    global diffids
+    global diffids treediffs
     global parents
     global diffcontext
     global diffencoding
@@ -6426,6 +6426,7 @@ proc mergediff {id} {
 
     set diffmergeid $id
     set diffids $id
+    set treediffs($id) {}
     # this doesn't seem to actually affect anything...
     set cmd [concat | git diff-tree --no-commit-id --cc -U$diffcontext $id]
     if {$limitdiffs && $vfilelimit($curview) ne {}} {
@@ -6445,7 +6446,7 @@ proc mergediff {id} {
 
 proc getmergediffline {mdf id np} {
     global diffmergeid ctext cflist mergemax
-    global difffilestart mdifffd
+    global difffilestart mdifffd treediffs
     global diffencoding
 
     $ctext conf -state normal
@@ -6462,6 +6463,7 @@ proc getmergediffline {mdf id np} {
 	    $ctext insert end "\n"
 	    set here [$ctext index "end - 1c"]
 	    lappend difffilestart $here
+	    lappend treediffs($id) $fname
 	    add_flist [list $fname]
 	    set diffencoding [get_path_encoding $fname]
 	    set l [expr {(78 - [string length $fname]) / 2}]