Integrate libgit2 for better table and string diff computations

[Duckwhale]

The current approach is hacky to say the least. For debugging failed assertions, a better diff output would be helpful.

Goals:

• Can use libgit2 APIs to compute "real" diffs in memory
• Failed assertions display (a part of) the diff string for tables and strings

Possibly relevant:

• https://libgit2.org/libgit2/#HEAD/group/diff/git_diff_buffers
• https://libgit2.org/libgit2/#HEAD/group/patch/git_patch_from_buffers
• https://libgit2.org/libgit2/#HEAD/group/diff/git_diff_from_buffer
• https://libgit2.org/libgit2/#HEAD/group/diff/git_diff_print

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Obviously, adding another dependency is hardly ideal. But I haven't found a simpler solution that "just works".

Additionally, relying on libgit2 instead of shelled-out git commands for the BuildTools module would make the changelog generation logic more portable, so that it could be used to implement a standalone changelog generator (for which I'd have other uses). Lastly, if a install (download/clone) command was to be implemented, it could also use this library.

[Duckwhale]

Example for some better output format (TBD):

https://github.com/avajs/ava#magic-assert

[Duckwhale]

While git would also enable cloning packages for #320 , neovim uses xdiff and that might be another option?

See https://github.com/neovim/neovim/blob/05fd70f19b39519985000141d022971e37d66189/src/nvim/lua/xdiff.c#L26