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
0 files updated, 0 files merged, 0 files removed, 0 files unresolved
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0 files updated, 0 files merged, 0 files removed, 0 files unresolved
adding b
created new head
0 files updated, 0 files merged, 0 files removed, 0 files unresolved
adding b
created new head