create-pull-request: Fix error on 2.0 versions of git

On 2.0 versions of git, the create-pull-request script exits with the
warning "No match for commit... Are you sure you pushed 'HEAD' there?".

This is due to a change in behavior where git used to guess the branch
you meant, but no longer does.  See the thread at
http://www.spinics.net/lists/git/msg233050.html for more information.

To accommodate the new behavior, if the COMMIT_ID is set to the default
of "HEAD", make it point explicitly to $BRANCH instead.

Signed-off-by: Ben Shelton <ben.shelton@ni.com>