Reword "your branch has diverged..." lines to reduce line length

Reword "your branch has diverged..." lines to reduce line length

The message length depends on the length of the branch name.  In my case,
the branch name "origin/add-chickens2" put the first line of the "your
branch has diverged" message over 80 characters, which triggered "less -FS"
to not exit automatically as expected.

This patch rewords the messages to make the lines generally shorter, so
that you'd need a significantly longer branch name to trigger the problem.

Signed-off-by: Avery Pennarun <apenwarr@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>