t6036: add lots of detail for directory/file conflicts in recursive case
There was a discussion of problematic directory/file conflicts with
virtual merge bases on the mailing list years ago at
https://public-inbox.org/git/AANLkTimwUQafGDrjxWrfU9uY1uKoFLJhxYs=vssOPqdf@mail.gmail.com/
Part of these corresponding tests made it into this testsuite. However,
the more problematic one didn't. And there are others that showcase the
problems even more. Add a very lengthy explanation, some of it from that
email, describing the tradeoffs in picking a recursive merge-base when
you're dealing with an add/add directory/file conflict.
The solution picked years ago is relatively good, but there is the
potential to do even better, assuming we're willing to pay a certain
performance cost.
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>