mq: qdiff with the same diff options than qrefresh (issue1350)

Specifically, if current patch is a git patch and mq.git=keep, diff in git
mode.

hgext/mq.py

@@ -771,11 +771,13 b' class queue(object):'
     def check_toppatch(self, repo):
         if len(self.applied) > 0:
             top = bin(self.applied[-1].rev)
+            patch = self.applied[-1].name
             pp = repo.dirstate.parents()
             if top not in pp:
                 raise util.Abort(_("working directory revision is not qtip"))
-            return top
-        return None
+            return top, patch
+        return None, None
+
     def check_localchanges(self, repo, force=False, refresh=True):
         m, a, r, d = repo.status()[:4]
         if m or a or r or d:
@@ -1125,7 +1127,7 b' class queue(object):'
             end = len(self.applied)
             rev = bin(self.applied[start].rev)
             if update:
-                top = self.check_toppatch(repo)
+                top = self.check_toppatch(repo)[0]
 
             try:
                 heads = repo.changelog.heads(rev)
@@ -1174,7 +1176,7 b' class queue(object):'
             wlock.release()
 
     def diff(self, repo, pats, opts):
-        top = self.check_toppatch(repo)
+        top, patch = self.check_toppatch(repo)
         if not top:
             self.ui.write(_("no patches applied\n"))
             return
@@ -1183,7 +1185,7 b' class queue(object):'
             node1, node2 = None, qp
         else:
             node1, node2 = qp, None
-        diffopts = self.diffopts(opts)
+        diffopts = self.diffopts(opts, patch)
         self.printdiff(repo, diffopts, node1, node2, files=pats, opts=opts)
 
     def refresh(self, repo, pats=None, **opts):
@@ -2058,7 +2060,7 b' def fold(ui, repo, *files, **opts):'
 
     if not files:
         raise util.Abort(_('qfold requires at least one patch name'))
-    if not q.check_toppatch(repo):
+    if not q.check_toppatch(repo)[0]:
         raise util.Abort(_('No patches applied'))
     q.check_localchanges(repo)
 

tests/test-mq-qdiff

@@ -2,6 +2,8 b''
 
 echo "[extensions]" >> $HGRCPATH
 echo "mq=" >> $HGRCPATH
+echo "[mq]" >> $HGRCPATH
+echo "git=keep" >> $HGRCPATH
 
 echo % init
 hg init a
@@ -60,3 +62,9 b' hg qdiff --nodates -w'
 
 echo % qdiff --reverse
 hg qdiff --nodates --reverse
+
+echo % qdiff preserve existing git flag
+hg qrefresh --git
+echo a >> lines
+hg qdiff
+

tests/test-mq-qdiff.out

@@ -103,3 +103,22 b' diff -r 35fb829491c1 lines'
  7
  8
  9
+% qdiff preserve existing git flag
+diff --git a/lines b/lines
+--- a/lines
++++ b/lines
+@@ -1,9 +1,12 @@
++
++
+ 1
+ 2
+ 3
+ 4
+-hello world
+-goodbye world
++hello  world
++     goodbye world
+ 7
+ 8
+ 9
++a