rebase: update comment about FreeBSD /bin/sh

rebase: update comment about FreeBSD /bin/sh

Commit 9f50d32 introduced a fix for FreeBSD /bin/sh misbehaviour
when dot-sourcing a file containing "return" statements outside of
any function, from a function in another shell script. That issue
affects FreeBSD 9.x, and is not present in the /bin/sh in FreeBSD
10.3 and later. Update the comment to clarify this.

The example from 9f50d32's commit message produces the expected output
on FreeBSD 10.3 and -CURRENT (the upcoming 11.0):

% sh script1.sh
only this line should show
%

Signed-off-by: Ed Maste <emaste@freebsd.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>