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 a
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
% should issue warning
adding a
created new head
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
% should not issue warning
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
marked working directory as branch testing
% should not issue warning