path-list: fix path-list-insert return value

path-list: fix path-list-insert return value

When path-list-insert is called on an existing path, it returned an
unrelated element in the list.  Luckily most of the callers are
ignoring the return value, but merge-recursive uses it at three places
and this would have resulted in a bogus rename detection.

Signed-off-by: Junio C Hamano <junkio@cox.net>