LZ4: vendoring in the kernel

We use something like git subtree at work, but not exactly (we evaluated subtree, but found it lacking for what we needed). What we do is keep a git mirror of the repo on our infra to which we add a for/$project branch. On this branch, we hold patches for integration into the build, fixes, symbol mangling patches, etc. Patches which should go upstream get issues opened for doing so.

To import, we take a subset of that repo (we usually do not care about docs and the like), put it in a tree, make a commit with its parent pointing to the previous commit (initial imports use a new root commit), and then merge into place using -Xsubtree. This allows us to keep the history as one would expect as well as not inflating our repo size with the full history of the import. Git checks ensure that the imported directory is only changed via this mechanism (and also protects against "evil merges").