Fix update_revision.sh to get updates from HEAD
Fix update_revision.sh to get updates from HEAD

After commit 8607a59, the script wasn't able to get updates from HEAD branch
of packages because it tried to get refs/heads/HEAD instead of HEAD.

Fix #1897

Signed-off-by: Erwan Prioul <9949cc68615082c237e6af925cacbd2ea7f2e953@linux.ibm.com>

advancetoolchain/advance-toolchain,advancetoolchain/advance-toolchain,advancetoolchain/advance-toolchain,advancetoolchain/advance-toolchain,advancetoolchain/advance-toolchain,advancetoolchain/advance-toolchain