update libgit2 submodule to recent main

[staktrace]

This is specifically to pick up the fix
at libgit2/libgit2#5839

I'd appreciate a new git2-rs release as well! (not super urgent though)

[staktrace]

Hm, looks like extprim-1.7.1 doesn't compile with the latest rustc nightly (2021-05-11). It compiled with an older nightly (I had 2020-12-14 prior to rustup update nightly).

[staktrace]

Put up kennytm/extprim#21 to fix extprim but I don't know if the maintainer is active. It's been almost a year since the last commit to that repo.

[alexcrichton]

Could CI be fixed by reducing the number of tests performed on nightly in that case?

[staktrace]

The dependency chain for extprim is like so:

systest/Cargo.toml :: build-dependencies -> ctest -> syntex_syntax2 -> extprim.

Since the dependency is required for building systest, we could presumably skip those tests on the nightly component of the build matrix and things would then work again. I'll put up another PR for that. However that's a short-term solution because presumably this change in rustc will eventually roll out to beta and release as well. It might be worth also considering migrating ctest to use something other than syntex, which is now deprecated (even the github repo is archived).

[staktrace]

#709 skips the systest for nightly rust.

[staktrace]

Rebased this PR on top of #709 so it's passing CI now.

[staktrace]

Thank you!