fix: support rust 1.54

[branchvincent]

This upgrades git2 to fix an incompatibility with rust 1.54

Stacktrace

error[E0495]: cannot infer an appropriate lifetime due to conflicting requirements
  --> /Users/brew/Library/Caches/Homebrew/cargo_cache/registry/src/github.heygears.com-1ecc6299db9ec823/git2-0.13.19/src/attr.rs:74:15
   |
74 |         match (self, other) {
   |               ^^^^^^^^^^^^^
   |
note: first, the lifetime cannot outlive the anonymous lifetime defined on the method body at 73:26...
  --> /Users/brew/Library/Caches/Homebrew/cargo_cache/registry/src/github.heygears.com-1ecc6299db9ec823/git2-0.13.19/src/attr.rs:73:26
   |
73 |     fn eq(&self, other: &AttrValue<'_>) -> bool {
   |                          ^^^^^^^^^^^^^
note: ...so that the types are compatible
  --> /Users/brew/Library/Caches/Homebrew/cargo_cache/registry/src/github.heygears.com-1ecc6299db9ec823/git2-0.13.19/src/attr.rs:74:15
   |
74 |         match (self, other) {
   |               ^^^^^^^^^^^^^
   = note: expected `(&AttrValue<'_>, &AttrValue<'_>)`
              found `(&AttrValue<'_>, &AttrValue<'_>)`
note: but, the lifetime must be valid for the lifetime `'_` as defined on the impl at 72:30...
  --> /Users/brew/Library/Caches/Homebrew/cargo_cache/registry/src/github.heygears.com-1ecc6299db9ec823/git2-0.13.19/src/attr.rs:72:30
   |
72 | impl PartialEq for AttrValue<'_> {
   |                              ^^
note: ...so that the types are compatible
  --> /Users/brew/Library/Caches/Homebrew/cargo_cache/registry/src/github.heygears.com-1ecc6299db9ec823/git2-0.13.19/src/attr.rs:79:16
   |
79 |             | (Self::Bytes(bytes), AttrValue::String(string)) => string.as_bytes() == *bytes,
   |                ^^^^^^^^^^^^^^^^^^
   = note: expected `AttrValue<'_>`
              found `AttrValue<'_>`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0495`.
error: failed to compile `cargo-instruments v0.4.1 (/private/tmp/cargo-instruments-20210801-36371-8u06ls/cargo-instruments-0.4.1)`, intermediate artifacts can be found at `/private/tmp/cargo-instruments-20210801-36371-8u06ls/cargo-instruments-0.4.1/target`

Caused by:
  could not compile `git2`

To learn more, run the command again with --verbose.

For more info, see

• rust change: Backwards-incompatible lifetime change (regression) rust-lang/rust#85574
• git2 fix: Update git2 to compile with latest rustc nightly rust-lang/git2-rs#712

[cmyr]

Tested manually, will fix up the clippy stuff separately. Thanks!