write_merge_state() writes out the merge heads, mode, and msg. But we
may want to write out heads, mode without the msg. So, split out heads
(+mode) into a separate function write_merge_heads() that is called by
write_merge_state().

No funtional change so far, except when these non-atomic writes are
interrupted: we write heads-mode-msg now when we used to write
heads-msg-mode.
Signed-off-by: NMichael J Gruber <git@grubix.eu>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>