mq: avoid data loss upon qfold + qmv (issue3058)
When renaming a patch A as B where B was previously qfolded into A and
therefore marked as removed, a versioned MQ would first restore B before
marking it as a copy of A, thus losing A changes. The undelete() call is
probably a left-over, wctx.copy() explicitely handles the case where the
destination is removed.
Also note that status command represents "hg rm b; hg mv a b" as:
A b
a
R a
which explains the first hunk in test-mq-qrename.t.