scripts/common-functions: fix sha_to_patch_in_branch
    
    sha_to_patch_in_branch f13abc1e8e1a3b7455511c4e122750127f6bc9b0 origin/SLE15-SP6
    returns
    origin/SLE15-SP6:patches.suse/watch_queue-fix-pipe-accounting-mismatch.patch
    
    which is obviously incorrect. We need to trim the branch name before
    filtering.