Git index version 3 support

[sebastianst]

Using git python to add files to an index fails at an index head assertion if the local repo is being otherwise worked on with git 2.46.0:

GitPython/git/index/fun.py

Lines 209 to 210 in cfadd9e

	# TODO: Handle version 3: extended data, see read-cache.c.
	assert version in (1, 2)

Is it planned to add support for version 3 soon?

I couldn't find any ticket tracking this, so apologies if this is a duplicate.

[Byron]

There is no plan to add support for extended index flag in V3, even though it would probably be quite easy to add if someone wanted to contribute support for it.

Generally, I probably wouldn't trust the index implementation as it is here and if possible, repo.git might be able to do the same with a decent amount of convenience.

[mgorny]

For the record, the workaround is to set:

git config --global feature.manyFiles false

and clone the repo again (or use -c feature.manyFiles=false when cloning it).

[tombedor]

Made a PR to make errors from using an unsupported version a bit easier to debug: #2055

[Byron]

It's a bit sad that the custom index implementation is half-baked only, and a little wonder that people can use it at all. In retrospect, it would have been better to only use Git for index manipulation. Maybe it's possible to workaround with repo.git as well.