templatekw: fix inconsistency of diffstat with diff.merge

Previously, `-T'{diffstat}'` was giving stats from the diff against p1,
regardless of whether `--config diff.merge=yes` is set.

This was inconsistent with `log --patch` which is aware of that option.

mercurial/templatekw.py

@@ -270,7 +270,7 b' def showdiffstat(context, mapping):'
     ui = context.resource(mapping, b'ui')
     ctx = context.resource(mapping, b'ctx')
     diffopts = diffutil.diffallopts(ui, {b'noprefix': False})
-    diff = ctx.diff(opts=diffopts)
+    diff = ctx.diff(diffutil.diff_parent(ctx), opts=diffopts)
     stats = patch.diffstatdata(util.iterlines(diff))
     maxname, maxtotal, adds, removes, binary = patch.diffstatsum(stats)
     return b'%d: +%d/-%d' % (len(stats), adds, removes)

tests/test-log.t

@@ -2001,6 +2001,8 b' Diff here should be the same:'
   @@ -0,0 +1,1 @@
   +b
   
+  $ hg log -r 3 -T'{diffstat}\n'
+  2: +2/-1
 
 Test that diff.merge is respected (file b was added on one side and
 and therefore merged cleanly)
@@ -2021,6 +2023,9 b' and therefore merged cleanly)'
   -b
   +c
   
+  $ hg log -r 3 -T'{diffstat}\n' --config diff.merge=yes
+  1: +1/-1
+
   $ cd ..
 
 'hg log -r rev fn' when last(filelog(fn)) != rev