file obstruction upon merging an already merged added/moved file (#4649)

Hi,

as per stsp's suggestion, sending this to the dev list.

I've reduced a problem we've been having with some merge operations on
our main repository from time to time down to the repro script attached
to this JIRA issue [1] (test_obstruction.bat).

Instead of a successful merge, which I'd expect in this case, I get the
following error instead when doing an svn merge [URL] -cX:
"local file obstruction, incoming file add upon merge"

The test case performs the following steps:
1. create test file and commit
2. modify the file twice and commit each modification separately
3. rename the file and commit
4. merge changes into the branch and commit
5. modify the file on trunk and commit
6. merge the branch back into trunk cherry picking the revision which
was created in step 4

At this point svn merge produces the error mentioned above.

To me this is unexpected, since I would expect the merge to succeed. In
the end all I'm doing is merging back the same file into trunk (even
though in trunk the file was modified).
This looks especially kind of inconsistent to me, since if I do either
of the following merges instead of a cherry picking merge, the merge
succeeds without producing any file conflicts:
- "svn merge [URL]" or
- "svn merge [URL]@X+1"

[1] https://issues.apache.org/jira/browse/SVN-4649

-- 
Regards,
Stefan Hett