mq: avoid data loss upon qfold + qmv (issue3058)

When renaming a patch A as B where B was previously qfolded into A and
therefore marked as removed, a versioned MQ would first restore B before
marking it as a copy of A, thus losing A changes. The undelete() call is
probably a left-over, wctx.copy() explicitely handles the case where the
destination is removed.

Also note that status command represents "hg rm b; hg mv a b" as:

  A b
    a
  R a

which explains the first hunk in test-mq-qrename.t.

hgext/mq.py

@@ -2614,8 +2614,6 b' def rename(ui, repo, patch, name=None, *'
                 r.dirstate.drop(patch)
                 r.dirstate.add(name)
             else:
-                if r.dirstate[name] == 'r':
-                    wctx.undelete([name])
                 wctx.copy(patch, name)
                 wctx.forget([patch])
         finally:

tests/test-mq-qrename.t

@@ -76,8 +76,8 b' Test overlapping renames (issue2388)'
   $ hg qrename patchb patchc
   $ hg qrename patcha patchb
   $ hg st --mq
-  M patchb
   M series
+  A patchb
   A patchc
   R patcha
   $ cd ..
@@ -94,3 +94,32 b' Test renames with mq repo (issue2097)'
   nothing changed
   [1]
   $ cd ..
+
+Test renaming to a folded patch (issue3058)
+
+  $ hg init issue3058
+  $ cd issue3058
+  $ hg init --mq
+  $ echo a > a
+  $ hg add a
+  $ hg qnew adda
+  $ echo b >> a
+  $ hg qnew addb
+  $ hg qpop
+  popping addb
+  now at: adda
+  $ hg ci --mq -m "save mq"
+  $ hg qfold addb
+  $ hg qmv addb
+  $ cat .hg/patches/addb
+  # HG changeset patch
+  # Parent 0000000000000000000000000000000000000000
+  
+  diff -r 000000000000 a
+  --- /dev/null	* (glob)
+  +++ b/a	* (glob)
+  @@ -0,0 +1,2 @@
+  +a
+  +b
+  $ cd ..
+