checkout/restore: refuse unmerging paths unless checking out of the index

checkout/restore: refuse unmerging paths unless checking out of the index

Recreating unmerged index entries using resolve-undo data,
recreating conflicted working tree files using unmerged index
entries, and writing data out of unmerged index entries, make
sense only when we are checking paths out of the index and not when
we are checking paths out of a tree-ish.

Add an extra check to make sure "--merge" and "--ours/--theirs"
options are rejected when checking out from a tree-ish, update the
document (especially the SYNOPSIS section) to highlight that they
are incompatible, and add a few tests to make sure the combination
fails.

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