When I wrote git_fast_filter.py, I was unaware of and did not forsee
libgit2.  So, although the license said the project could be used under
whatever license git.git was, there was still a potential barrier for
usage by libgit2.  I'm not sure if libgit2 will ever want to use
filter-repo, but I don't want the barrier there and I would like to
avoid a repeat of this problem.  (Also, since filter-repo is for the
most part a one-shot usage tool, I doubt that the normal copyleft
provisions could provide much value.)

MIT is widely used, compatible with just about everyting, and is
preferred by Palantir (my current employer) for open source
contributions.  So, I contacted all other contributors (Jim is still at
Sandia) and got permission to relicense.

Signed-off-by: Elijah Newren <newren@gmail.com>