fix qrefresh'ing an empty patch

This is not superefficient (the commit will have to walk the whole tree),
but it works.

hgext/mq.py

@@ -1015,10 +1015,10 b' class queue:'
             m = util.unique(mm)
             r = util.unique(dd)
             a = util.unique(aa)
-            filelist = filter(matchfn, util.unique(m + r + a))
+            c = [filter(matchfn, l) for l in (m, a, r, [], u)]
+            filelist = util.unique(c[0] + c[1] + c[2])
             patch.diff(repo, patchparent, files=filelist, match=matchfn,
-                       fp=patchf, changes=(m, a, r, [], u),
-                       opts=self.diffopts())
+                       fp=patchf, changes=c, opts=self.diffopts())
             patchf.close()
 
             repo.dirstate.setparents(*cparents)
@@ -1066,7 +1066,8 b' class queue:'
                 message = msg
 
             self.strip(repo, top, update=False, backup='strip', wlock=wlock)
-            n = repo.commit(filelist, message, changes[1], force=1, wlock=wlock)
+            n = repo.commit(filelist, message, changes[1], match=matchfn,
+                            force=1, wlock=wlock)
             self.applied[-1] = statusentry(revlog.hex(n), patchfn)
             self.applied_dirty = 1
         else:

tests/test-mq

@@ -55,6 +55,18 b' sed -e "s/^\\(diff -r \\)\\([a-f0-9]* \\)/\\1'
     -e "s/\(+++ [a-zA-Z0-9_/.-]*\).*/\1/" \
     -e "s/\(--- [a-zA-Z0-9_/.-]*\).*/\1/" .hg/patches/test.patch
 
+echo % empty qrefresh
+
+hg qrefresh -X a
+echo 'revision:'
+hg diff -r -2 -r -1
+echo 'patch:'
+cat .hg/patches/test.patch
+echo 'working dir diff:'
+hg diff --nodates -q
+# restore things
+hg qrefresh
+
 echo % qpop
 
 hg qpop

tests/test-mq.out

@@ -71,6 +71,17 b' diff -r xa'
 @@ -1,1 +1,2 @@ a
  a
 +a
+% empty qrefresh
+revision:
+patch:
+foo bar
+
+working dir diff:
+--- a/a
++++ b/a
+@@ -1,1 +1,2 @@ a
+ a
++a
 % qpop
 Patch queue now empty
 % qpush