Bump libgit2 to 1.4.0

[dtolnay]

https://github.com/libgit2/libgit2/releases/tag/v1.4.0 was just released and contains libgit2/libgit2#6203, which we need for unblocking fast fetches in Cargo: rust-lang/cargo#10078.

I tried naïvely bumping the libgit2 submodule like in #751 but cargo test has some test failures and several segfaults, so this may be a high-effort upgrade.

[alexcrichton]

Typically this comes about because structures in C changed, but this can be tested with cargo run in the systest directory which should print out which structs in Rust are different than those in C. Once the libgit2-sys crate is adjusted that should probably weed out the segfaults.