Merge tests

[Greg Ward]

Hi all --

I'm reading through test-merge*, trying to find a good place to test
the various "can't merge with ancestor" cases since I want to improve
the error messages.  Some of the older ones are a bit confusing:

  * test-merge2: appears to be testing the merge with working dir case
when files are removed, but that's unclear. A comment would be nice.
  * test-merge3: contains no "merge" or "update" commands, so it can't
possibly be doing a merge.  (Right?)

Some of the other test-merge[0-9] scripts are also a little
underdocumented, although they're not too hard to figure out with a
little hg log'ing.  It would be nice to have comments right in the
scripts, though.  So if some can explain what's up with
test-merge{2,3}, I'll see if I can add some useful comments.

Incidentally, I don't see a good place to test the "abort: can't merge
with ancestor" errors, so I guess I'll add a new test script.

Greg