[PATCH] gitk: Heed the lines of context in merge commits

There is an edit box where the number of context lines can be chosen.
But it was only used when regular diffs were displayed, not for
merge commits.   This fixes it.

Signed-off-by: Johannes Sixt <johannes.sixt@telecom.at>
Signed-off-by: Paul Mackerras <paulus@samba.org>

gitk

@@ -5032,13 +5032,14 @@ proc getblobline {bf id} {
 proc mergediff {id l} {
     global diffmergeid mdifffd
     global diffids
+    global diffcontext
     global parentlist
     global limitdiffs viewfiles curview
 
     set diffmergeid $id
     set diffids $id
     # this doesn't seem to actually affect anything...
-    set cmd [concat | git diff-tree --no-commit-id --cc $id]
+    set cmd [concat | git diff-tree --no-commit-id --cc -U$diffcontext $id]
     if {$limitdiffs && $viewfiles($curview) ne {}} {
 	set cmd [concat $cmd -- $viewfiles($curview)]
     }