filter-repo: re-license

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 <[email protected]>

Author: newren

COPYING

@@ -1,356 +1,23 @@
-The files in this project are provided under the same license as git.git
-(GPLv2 as of writing; in terms of the git.git licensing preamble, this means
-I want to automatically adopt any license change made by git.git, and expect
-any contributors to this project to agree to have their contributions
-automatically relicensed if git.git is.)
+git-filter-repo itself and most the files in this repository (exceptions
+noted below) are provided under the MIT license (see COPYING.mit).
 
-Some items from git.git have been copied, modified, and included in the
-files here for convenience, including some files that are nearly wholesale
-copies.  I hope I have kept the appropriate attributions, but it's possible
-I may have missed a couple.  Please let me know of any corrections you
-notice are needed.
+The usage of the MIT license probably makes filter-repo compatible with
+everything, but just in case, these files can also be used under whatever
+open source license[1] that git.git or libgit2 use now or in the future
+(currently GPL[2] and GPL-with-linking-exception[3]).  Further, the
+examples (in contrib/filter-repo-demos/ and t/t9391/) can also be used
+under the same license that libgit2 provides their examples under (CC0,
+currently[4]).
 
-A copy of GPLv2 is included below.
+Exceptions:
 
-----------------------------------------
+  - The test harness (t/test-lib.sh, t/test-lib-functions.sh) is a slightly
+    modified copy of git.git's test harness (the difference being that my
+    copy doesn't require a built version of 'git' to be present).  These
+    are thus GPL2 (see COPYING.gpl), and are individually marked as such.
 
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+[1] ...as defined by the Open Source Initiative (https://opensource.org/)
+[2] https://git.kernel.org/pub/scm/git/git.git/tree/COPYING
+[3] https://github.com/libgit2/libgit2/blob/master/COPYING
+[4] https://github.com/libgit2/libgit2/blob/master/examples/COPYING