Update git2 requirement from 0.7.5 to 0.8.0

Updates the requirements on [git2](https://github.com/alexcrichton/git2-rs) to permit the latest version.
- [Release notes](https://github.com/alexcrichton/git2-rs/releases)
- [Commits](https://github.com/alexcrichton/git2-rs/commits/git2-curl-0.8.0)

Signed-off-by: dependabot[bot] <support@dependabot.com>

Cargo.toml

@@ -31,7 +31,7 @@ failure = "0.1.2"
 filetime = "0.2"
 flate2 = { version = "1.0.3", features = ['zlib'] }
 fs2 = "0.4"
-git2 = "0.7.5"
+git2 = "0.8.0"
 git2-curl = "0.8.1"
 glob = "0.2.11"
 hex = "0.3"