rerere: write out each record of MERGE_RR in one go

rerere: write out each record of MERGE_RR in one go

Instead of writing the hash for a conflict, a HT, and the path
with three separate write_in_full() calls, format them into a
single record into a strbuf and write it out in one go.

As a more recent "rerere remaining" codepath abuses the .util field
of the merge_rr data to store a sentinel token, make sure that
codepath does not call into this function (of course, "remaining" is
a read-only operation and currently does not call it).

Signed-off-by: Junio C Hamano <gitster@pobox.com>