mq: support "qimport --existing --name renametothis thatexistingpatch"

Before this change, the command would abort with a not too clear "patch
renametothis does not exist" error.

This change makes:
  qimport --existing --name renametothis thatexistingpatch
equivalent to:
  qimport --existing thatexistingpatch; qrename thatexistingpatch renametothis

hgext/mq.py

@@ -1687,11 +1687,22 b' class queue(object):'
             if existing:
                 if filename == '-':
                     raise util.Abort(_('-e is incompatible with import from -'))
-                if not patchname:
-                    patchname = normname(filename)
-                self.check_reserved_name(patchname)
-                if not os.path.isfile(self.join(patchname)):
-                    raise util.Abort(_("patch %s does not exist") % patchname)
+                filename = normname(filename)
+                self.check_reserved_name(filename)
+                originpath = self.join(filename)
+                if not os.path.isfile(originpath):
+                    raise util.Abort(_("patch %s does not exist") % filename)
+
+                if patchname:
+                    self.check_reserved_name(patchname)
+                    checkfile(patchname)
+
+                    self.ui.write(_('renaming %s to %s\n')
+                                        % (filename, patchname))
+                    os.rename(originpath, self.join(patchname))
+                else:
+                    patchname = filename
+
             else:
                 try:
                     if filename == '-':

tests/test-mq-qimport

@@ -109,3 +109,12 b' hg up -C'
 hg qimport --push another.diff
 hg qfin -a
 hg qimport -rtip -P
+
+hg qpop -a
+hg qdel -k 2.diff
+echo % qimport -e
+hg qimport -e 2.diff
+hg qdel -k 2.diff
+echo % qimport -e --name newname oldexisitingpatch
+hg qimport -e --name this-name-is-better 2.diff
+hg qser

tests/test-mq-qimport.out

@@ -52,3 +52,12 b' applying another.diff'
 now at: another.diff
 patch b.diff finalized without changeset message
 patch another.diff finalized without changeset message
+popping 2.diff
+patch queue now empty
+% qimport -e
+adding 2.diff to series file
+% qimport -e --name newname oldexisitingpatch
+renaming 2.diff to this-name-is-better
+adding this-name-is-better to series file
+this-name-is-better
+url.diff