create-pull-request: fix for newer git

create-pull-request: fix for newer git

Fixed when git > 2.1.0:
$ ./scripts/create-pull-request -r HEAD^ -u contrib -b rbt/git
fatal: Not a valid revision: rbt/git
ERROR: git request-pull reported an error

This is because newer git requires both local and remote branch named as
rbt/git, but usually, we only named the remote branch as rbt/foo, and
foo for local branch.

Add a option '-l' to fix the problem, default is HEAD.

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>