apply --allow-overlap: fix a corner case

apply --allow-overlap: fix a corner case

Yes, yes, this is supposed to be only a band-aid option for `git add -p`
not Doing The Right Thing. But as long as we carry the `--allow-overlap`
option, we might just as well get it right.

This fixes the case where one hunk inserts a line before the first line,
and is followed by a hunk whose context overlaps with the first one's
and which appends a line at the end.

Signed-off-by: Johannes Schindelin <redacted>
Signed-off-by: Junio C Hamano <redacted>