Enable ref log creation in git checkout -b.

Enable ref log creation in git checkout -b.

Switch git checkout -b to use git-update-ref rather than echo and
a shell I/O redirection.  This is more in line with typical GIT
commands and allows -b to be logged according to the normal ref
logging rules.

Added -l option to allow users to create the ref log at the same
time as creating a branch.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>