checkout: print a message when switching unborn branches

checkout: print a message when switching unborn branches

When we switch to a new branch using checkout, we usually output a
message indicating what happened. However, when we switch from an unborn
branch to a new branch, we do not print anything, which may leave the
user wondering what happened.

The reason is that the unborn branch is a special case (see abe1998),
and does not follow the usual switch_branches code path. Let's add a
similar informational message to the special case to match the usual
code path.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>