rerere: factor out handle_conflict function

rerere: factor out handle_conflict function

Factor out the handle_conflict function, which handles a single
conflict in a path.  This is in preparation for a subsequent commit,
where this function will be re-used.

Note that this does change the behaviour of 'git rerere' slightly.
Where previously we'd consider all files where an unmatched conflict
marker is found as invalid, we now only consider files invalid when
the "ours" conflict marker ("<<<<<<< <text>") is unmatched, not when
other conflict markers (e.g. "=======") is unmatched.

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