rerere: split code to call ll_merge() further

The merge() helper function is given an existing rerere ID (i.e. the
name of the .git/rr-cache/* subdirectory, and the variant number)
that identifies one <preimage, postimage> pair, try to see if the
conflicted state in the given path can be resolved by using the pair,
and if this succeeds, then update the conflicted path with the
result in the working tree.

To implement rerere_forget() in the multiple variant world, we'd
need a helper to do the "see if a <preimage, postimage> pair cleanly
resolves a conflicted state we have in-core" part, without actually
touching any file in the working tree, in order to identify which
variant(s) to remove.  Split the logic to do so into a separate
helper function try_merge() out of merge().

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