rerere: only return whether a path has conflicts or not

rerere: only return whether a path has conflicts or not

We currently return the exact number of conflict hunks a certain path
has from the 'handle_paths' function.  However all of its callers only
care whether there are conflicts or not or if there is an error.
Return only that information, and document that only that information
is returned.  This will simplify the code in the subsequent steps.

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