Squashed 'extern/common/' content from commit 5844c46

git-subtree-dir: extern/common
git-subtree-split: 5844c4627682f009fff8fc048793879c0f0a3064
Merge commit 'af3aa666e3643967b43d76530cb9f93a46409e1f' as 'extern/common'
Add script for pulling in changes from external repos