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
abort: repository a not found!
255
abort: error: Connection refused
255
abort: repository a not found!
255
abort: Permission denied: ../a
255
abort: repository a not found!
255
destination directory: q
abort: destination 'q' is not empty
abort: destination 'a' is not empty
255
adding c
255
dir is still here
repo is gone