Migrating merge based project from legacy git tree

[Ed Maste]

Downstream FreeBSD projects will often use a merge based approach for
staying up-to-date with new FreeBSD changes. That is, local work is
committed to their fork, and FreeBSD updates arrive via something like
"git merge upstream/master". These continue, resulting in a history of
mixed local work and upstream merges.

With FreeBSD's transition to a Git repository the Git hashes will
change from the current GitHub mirror, in order to correct some issues
with the existing svn2git conversion that would otherwise cause grief
(in particular, related to contrib/ code management/updates). One of
the deliverables from the Git Working Group is a documented process
for migrating a downstream fork from "old" to "new" hashes.

Gleb has documented that process at
https://github.com/freebsd/git_conv/wiki/Migrating-merge-based-project-from-legacy-git-tree.
I would appreciate it if merged-based downstream FreeBSD consumers can
give this a try, and provide feedback. Note that the hashes in the
beta repository will change at least once before it is finalized, so
try this on an experimental basis for now.

This document will move to a permanent home later in the transition process.