clonebundles: rewrite documentation There are a lot of considerations server operators need to know before deploying clone bundles. They should be documented. So I rewrote the extension docs to contain this information.

exchange: support streaming clone bundles in clone bundles Now that we have support for detecting compatible stream clone bundles in bundle specifications, we can safely add support for applying stream clone bundles to the clone bundles feature.

exchange: parse requirements from stream clone specification string Stream clone bundles can only be consumed if the consumer supports the exact format requirements that were present on the producer. This patch adds support for encoding and verifying the format requirements on the bundle specification string for a stream clone bundle are supported by the local repository. If they aren't, we raise an UnsupportedBundleSpecification, just like we do when an unknown compression or bundle type is encountered. The impetus for this patch is so the clone bundles manifest can advertise stream clone bundles and so clients can filter out stream clones with unsupported format requirements. e.g. a stream clone produced with the not-yet-invented "revlogv2" format will be ignored by clients that only support "revlogv1."

exchange: support parameters in bundle specification strings Sometimes a basic type string is not sufficient for representing the contents of a bundle. Take bundle2 for example: future bundle2 files may contain parts that today's bundle2 parser can't read. Another example is stream clone data. These require clients to support specific repository formats or they won't be able to read the written files. In both scenarios, we need to describe additional metadata beyond the outer container type. Furthermore, this metadata behaves more like an unordered set, so an order-based declaration format (such as static strings) is not sufficient. We introduce support for "parameters" into the bundle specification string. These are essentially key-value pairs that can be used to encode additional metadata about the bundle. Semicolons are used as the delimiter partially to increase similarity to MIME parameter values (see RFC 2231) and because they are relatively safe from the command line (although values will need quotes to avoid interpretation as multiple shell commands). Alternatives considered were spaces (a bit annoying to encode) and '&' (similar to URL query strings) (which will do bad things in a shell if unquoted). The parsing function now returns a dict of parsed parameters and consumers have been updated accordingly.

commands: support consuming stream clone bundles For the same reasons that we don't produce stream clone bundles with `hg bundle`, we don't support consuming stream clone bundles with `hg unbundle`. We introduce a complementary debug command for applying stream clone bundles. This command is mostly to facilitate testing. Although it may be used to manually apply stream clone bundles until a more formal mechanism is (possibly) adopted.

commands: support creating stream clone bundles Now that we have support for recognizing the streaming clone bundle type, add a debug command for creating them. I decided to create a new debug command instead of adding support to `hg bundle` because stream clone bundles are not exactly used the same way as normal bundle files and I don't want to commit to supporting them through the official `hg bundle` command forever. A debug command, however, can be changed without as much concern for backwards compatibility. As part of this, `hg bundle` will explicitly reject requests to produce stream bundles. This command will be required by server operators using stream clone bundles with the clone bundles feature.

exchange: support for streaming clone bundles Now that we have a mechanism to produce and consume streaming clone bundles, we need to teach the human-facing bundle specification parser and the internal bundle file header reading code to be aware of this new format. This patch does so. For the human-facing bundle specification, we choose the name "packed" to describe "streaming clone bundles" because the bundle is essentially a "pack" of raw revlog files that are "packed" together. There should probably be a bikeshed over the name, especially since it is human facing.

streamclone: support for producing and consuming stream clone bundles Up to this point, stream clones only existed as a dynamically generated data format produced and consumed during streaming clones. In order to support this efficient cloning format with the clone bundles feature, we need a more formal, on disk representation of the streaming clone data. This patch introduces a new "bundle" type for streaming clones. Unlike existing bundles, it does not contain changegroup data. It does, however, share the same concepts like the 4 byte header which identifies the type of data that follows and the 2 byte abbreviation for compression types (of which only "UN" is currently supported). The new bundle format is essentially the existing stream clone version 1 data format with some headers at the beginning. Content negotiation at stream clone request time checked for repository format/requirements compatibility before initiating a stream clone. We can't do active content negotiation when using clone bundles. So, we put this set of requirements inside the payload so consumers have a built-in mechanism for checking compatibility before reading and applying lots of data. Of course, we will also advertise this requirements set in clone bundles. But that's for another patch. We currently don't have a mechanism to produce and consume this new bundle format. This will be implemented in upcoming patches. It's worth noting that if a legacy client attempts to `hg unbundle` a stream clone bundle (with the "HGS1" header), it will abort with: "unknown bundle version S1," which seems appropriate.