rpi-update incorrectly detects latest git commit when called with BRANCH=next

"BRANCH=next rpi-update" will always fetch the latest git commit from the next branch, and re-downloads everything even if the local firmware git repo is up to date (which is annoying since rpi-update from the master branch of the firmware repo doesn't have this problem). The root cause is that the master branch is hardcoded into the git revision check:

GITREV=$(git ls-remote -h ${REPO_URI} refs/heads/master | awk '{print $1}')

This pull request fixes this issue (I tested with UPDATE_SELF=0 after modifying the script, and it works). Thanks!