mq: fix opts['unified'] = '0' to take effect when qdiff --stat

`opts['unified'] = '0'` tries to reduce the number of lines to be
processed by patch.diffstat, but here it does not affect diffopts,
thus it does not make sense.

Instead, we can change `diffopts.context` directly.

Added test case for qdiff --stat.

hgext/mq.py

@@ -477,8 +477,6 b' class queue(object):'
     def printdiff(self, repo, diffopts, node1, node2=None, files=None,
                   fp=None, changes=None, opts={}):
         stat = opts.get('stat')
-        if stat:
-            opts['unified'] = '0'
 
         m = cmdutil.match(repo, files, opts)
         if fp is None:
@@ -487,6 +485,7 b' class queue(object):'
             def write(s, **kw):
                 fp.write(s)
         if stat:
+            diffopts.context = 0
             width = self.ui.interactive() and util.termwidth() or 80
             chunks = patch.diff(repo, node1, node2, m, changes, diffopts)
             for chunk, label in patch.diffstatui(util.iterlines(chunks),

tests/test-mq-qdiff

@@ -68,3 +68,5 b' hg qrefresh --git'
 echo a >> lines
 hg qdiff
 
+echo % qdiff --stat
+hg qdiff --stat

tests/test-mq-qdiff.out

@@ -122,3 +122,6 b' diff --git a/lines b/lines'
  8
  9
 +a
+% qdiff --stat
+ lines |  7 +++++--
+ 1 files changed, 5 insertions(+), 2 deletions(-)