rerere: refactor "replay" part of do_plain_rerere()

rerere: refactor "replay" part of do_plain_rerere()

Extract the body of a loop that attempts to replay recorded
resolution for each conflicted path into a helper function, not
because I want to call it from multiple places later, but because
the logic has become too deeply nested and hard to read.

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