The "fast import" format originates with Git, where it is used as
the generic backend for converting other SCM repositories to git.
It's generic enough that it can be used to feed a Mercurial repository
as well, and that's what this extension does: read a "fast import"
stream and turn it into changesets in a Mercurial repository. The
potential of this extension is that any foreign SCM that can be
converted to Git can also be converted to Mercurial, regardless of
the capabilities of Mercurial's own ConvertExtension. The catch is
that the conversion is a little cumbersome: first convert to a
fast-import dump, then feed that dump to Mercurial.

Homepage:
https://wiki.mercurial-scm.org/FastImportExtension
