merge-ort: store more specific conflict information

It is all fine and dandy for a regular Git command that is intended to
be run interactively to produce a bunch of messages upon an error.

However, in `merge-ort`'s case, we want to call the command e.g. in
server-side software, where the actual error messages are not quite as
interesting as machine-readable, immutable terms that describe the exact
nature of any given conflict.

With this patch, the `merge-ort` machinery records the exact type (as
specified via an `enum` value) as well as the involved path(s) together
with the conflict's message.

Signed-off-by: Elijah Newren <[email protected]>
Signed-off-by: Johannes Schindelin <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: newren