A bundle is a container for repository data.
    Bundles are used as standalone files as well as the interchange format
    over the wire protocol used when two Mercurial peers communicate with
    each other.
    Bundles produced since Mercurial 0.7 (September 2005) have a 4 byte
    header identifying the major bundle type. The header always begins with
    ``HG`` and the follow 2 bytes indicate the bundle type/version. Some
    bundle types have additional data after this 4 byte header.
    The following sections describe each bundle header/type.
    ``HG10`` headers indicate a *changegroup bundle*. This is the original
    bundle format, so it is sometimes referred to as *bundle1*. It has been
    present since version 0.7 (released September 2005).
    This header is followed by 2 bytes indicating the compression algorithm
    used for data that follows. All subsequent data following this
    compression identifier is compressed according to the algorithm/method
    Supported algorithms include the following.
       *bzip2* compression.
       Bzip2 compressors emit a leading ``BZ`` header. Mercurial uses this
       leading ``BZ`` as part of the bundle header. Therefore consumers
       of bzip2 bundles need to *seed* the bzip2 decompressor with ``BZ`` or
       seek the input stream back to the beginning of the algorithm component
       of the bundle header so that decompressor input is valid. This behavior
       is unique among supported compression algorithms.
       Supported since version 0.7 (released December 2006).
      *zlib* compression.
       Supported since version 0.9.2 (released December 2006).
      *Uncompressed* or no compression. Unmodified changegroup data follows.
      Supported since version 0.9.2 (released December 2006).
    3rd party extensions may implement their own compression. However, no
    authority reserves values for their compression algorithm identifiers.
    ``HG2X`` headers (where ``X`` is any value) denote a *bundle2* bundle.
    Bundle2 bundles are a container format for various kinds of repository
    data and capabilities, beyond changegroup data (which was the only data
    supported by ``HG10`` bundles.
    ``HG20`` is currently the only defined bundle2 version.
    The ``HG20`` format is not yet documented here. See the inline comments
    in ``mercurial/exchange.py`` for now.
    Initial ``HG20`` support was added in Mercurial 3.0 (released May
    2014). However, bundle2 bundles were hidden behind an experimental flag
    until version 3.5 (released August 2015), when they were enabled in the
    wire protocol. Various commands (including ``hg bundle``) did not
    support generating bundle2 files until Mercurial 3.6 (released November
    A ``HGS1`` header indicates a *streaming clone bundle*. This is a bundle
    that contains raw revlog data from a repository store. (Typically revlog
    data is exchanged in the form of changegroups.)
    The purpose of *streaming clone bundles* are to *clone* repository data
    very efficiently.
    The ``HGS1`` header is always followed by 2 bytes indicating a
    compression algorithm of the data that follows. Only ``UN``
    (uncompressed data) is currently allowed.
    ``HGS1UN`` support was added as an experimental feature in version 3.6
    (released November 2015) as part of the initial offering of the *clone
    bundles* feature.