Update git2 requirement from 0.14.2 to 0.15.0 in /boa_tester

Updates the requirements on [git2](https://github.com/rust-lang/git2-rs) to permit the latest version.
- [Release notes](https://github.com/rust-lang/git2-rs/releases)
- [Commits](rust-lang/git2-rs@0.14.2...git2-curl-0.15.0)

---
updated-dependencies:
- dependency-name: git2
  dependency-type: direct:production
...

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

boa_tester/Cargo.toml

@@ -22,7 +22,7 @@ regex = "1.5.5"
 once_cell = "1.10.0"
 colored = "2.0.0"
 fxhash = "0.2.1"
-git2 = "0.14.2"
+git2 = "0.15.0"
 hex = "0.4.3"
 num-format = "0.4.0"
 gc = { version = "0.4.1", features = ["derive"] }