merge-recursive: add a bunch of FIXME comments documenting known bugs

merge-recursive: add a bunch of FIXME comments documenting known bugs

The plan is to just delete merge-recursive, but not until everyone is
comfortable with merge-ort as a replacement.  Given that I haven't
switched all callers of merge-recursive over yet (e.g. git-am still uses
merge-recursive), maybe there's some value documenting known bugs in the
algorithm in case we end up keeping it or someone wants to dig it up in
the future.

Signed-off-by: Elijah Newren <newren@gmail.com>
Reviewed-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>