lfs: autoload the extension when cloning from repo with lfs enabled This is based on a patch by Gregory Szorc. I made small adjustments to clean up the messaging when the server has the extension enabled, but the client has it disabled (to prevent autoloading). Additionally, I added a second server capability to distinguish between the server having the extension enabled, and the server having LFS commits. This helps prevent unnecessary requirement propagation- the client shouldn't add a requirement that the server doesn't have, just because the server had the extension loaded. The TODO I had about advertising a capability when the server can natively serve up blobs isn't relevant anymore (we've had 2 releases that support this), so I dropped it. Currently, we lazily add the "lfs" requirement to a repo when we first encounter LFS data. Due to a pretxnchangegroup hook that looks for LFS data, this can happen at the end of clone. Now that we have more control over how repositories are created, we can do better. This commit adds a repo creation option to add the "lfs" requirement. hg.clone() sets this creation option if the remote peer is advertising lfs usage (as opposed to just support needed to push). So, what this change effectively does is have cloned repos automatically inherit the "lfs" requirement. Differential Revision: https://phab.mercurial-scm.org/D5130