git-branch: default to --track

git-branch: default to --track

"git branch --track" will setup config variables when branching from
a remote branch, so that if you say "git pull" while being on that
branch, it automatically fetches the correct remote, and merges the
correct branch.

Often people complain that this is not the default for "git branch".
Make it so.

Signed-off-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>