Update git2 requirement from 0.19 to 0.20

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

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

tools/proto-compiler/Cargo.toml

@@ -7,7 +7,7 @@ publish      = false
 rust-version = "1.60"
 
 [dependencies]
-git2                         = "0.19"
+git2                         = "0.20"
 prost-build                  = "0.13"
 walkdir                      = "2.3"
 argh                         = "0.1"