tests/test-issue842

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

#!/bin/sh

hg init test
cd test
echo foo > a
hg ci -Ama

hg up -r0000
echo bar > a
echo % should issue warning
hg ci -Amb

hg up -r0000
echo stuffy > a
echo % should not issue warning
hg ci -q -Amc

hg up -r0000
echo crap > a
hg branch testing
echo % should not issue warning
hg ci -q -Amd