[Bug] Should allow to merge into mixed-revision working copy when specifying --ignore-ancestry

If you merge "the old way", i.e. specifying --ignore-ancestry,
Subversion sometimes requires you to update your WC:

svn: E195020: Cannot merge into mixed-revision working copy
[xxxxx:xxxxx]; try updating first

It wasn't the case for 1.6, and thus may be considered a regression.

Current behavior is very annoying when doing multiple merge-commit
operations, especially into large working copy. In such case, I *know*
that I have up-to-date WC after each commit, but I still have to waste
time updating.

Best wishes,
Andrey Paramonov

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