tests/test-gpg.out

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

adding foo
% no signatures
% hg sign 0
Signing 0:e63c23eaa88a
% hg sigs
hgtest                             0:e63c23eaa88ae77967edcf4ea194d31167c478b0
% hg sigcheck 0
e63c23eaa88a is signed by:
 hgtest