Allow checkout -B <current-branch> to update the current branch

Allow checkout -B <current-branch> to update the current branch

When on master, "git checkout -B master <commit>" is a more natural way to
say "git reset --keep <commit>", which was originally invented for the
exact purpose of moving to the named commit while keeping the local changes
around.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>