[merge tracking] Recording merge info when part of the merge is skipped

As a follow-up to r21504, I fixed merge_tests.py 7 and 8 in r21511
(basically by reverting earlier r20438, which made the tests incorrect
now that the code is closer to being correct).

Test 17 seems to be a little more tricky, as part of its merge
succeeds, and part of it is skipped.

Under dir "C", we create unversioned file "foo". Then we merge a tree
("F") which contains versioned file "foo" and some other stuff into
"C":

WC:

---
C/
  foo
Merge source:
-------------
F/
  foo
  Q/
    bar
"foo" is skipped (because it's obstructed), while "Q" and "Q/bar" are
merged successfully.  Where in the WC should the merge info be
recorded?
My feeling is that merge info should apply to "Q" (and thus implicity
to "bar", by inheritance).  It should not apply to "foo" (because it
was not merged).  However, what about "C", which had partial merge of
r2 succeed?  Because part of the merge was skipped, I tend to think
that it should not have merge info recorded on it.
Thoughts?