sha1-name: check for overflow of N in "foo^N" and "foo~N"

sha1-name: check for overflow of N in "foo^N" and "foo~N"

Reject values that don't fit into an int, as get_parent() and
get_nth_ancestor() cannot handle them.  That's better than potentially
returning a random object.

If this restriction turns out to be too tight then we can switch to a
wider data type, but we'd still have to check for overflow.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>