Bump git2 from 0.13.20 to 0.13.21

Bumps [git2](https://github.com/rust-lang/git2-rs) from 0.13.20 to 0.13.21.
- [Release notes](https://github.com/rust-lang/git2-rs/releases)
- [Commits](rust-lang/git2-rs@0.13.20...0.13.21)

---
updated-dependencies:
- dependency-name: git2
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

Cargo.lock

@@ -20,7 +20,7 @@ hyper_cgi = "0.1.10"
 hyper-staticfile = "0.6.0"
 hyper-tls = "0.5.0"
 toml = "0.5.8"
-git2 = "0.13.20"
+git2 = "0.13.21"
 tracing = { version = "0.1.26", features = ["max_level_trace", "release_max_level_trace"] }
 tracing-futures = "0.2.5"
 tracing-subscriber = "0.2.19"