rerere: return strbuf from handle path

rerere: return strbuf from handle path

Currently we write the conflict to disk directly in the handle_path
function.  To make it re-usable for nested conflicts, instead of
writing the conflict out directly, store it in a strbuf and let the
caller write it out.

This does mean some slight increase in memory usage, however that
increase is limited to the size of the largest conflict we've
currently processed.  We already keep one copy of the conflict in
memory, and it shouldn't be too large, so the increase in memory usage
seems acceptable.

As a bonus this lets us get replace the rerere_io_putconflict function
with a trivial two line function.

Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>