merge & sequencer: unify codepaths that write "Conflicts:" hint

merge & sequencer: unify codepaths that write "Conflicts:" hint

Two identical loops in suggest_conflicts() in merge, and
do_recursive_merge() in sequencer, can use a single helper function
extracted from the latter that prepares the "Conflicts:" hint that
is meant to remind the user the paths for which merge conflicts had
to be resolved to write a better commit log message.

Signed-off-by: Junio C Hamano <redacted>