bundle2: client side support for a part to import external bundles Bundle2 opens doors to advanced features allowing to reduce load on mercurial servers, and improve clone experience for users on unstable or slow networks. For instance, it could be possible to pre-generate a bundle of a repository, and give a pointer to it to clients cloning the repository, followed by another changegroup with the remainder. For significantly big repositories, this could come as several base bundles with e.g. 10k changesets, which, combined with checkpoints (not part of this change), would prevent users with flaky networks from starting over any time their connection fails. While the server-side support for those features doesn't exist yet, it is preferable to have client-side support for this early-on, allowing experiments on servers only requiring a vanilla client with bundle2 enabled.