test(git): Clean up shallow fetch tests (#15002)

### What does this PR try to resolve?

When looking to add shallow fetch for the git CLI, I found it hard to
tell what we test. This tries to clean up the tests and make it easier
to add new ones. My hope is to later generalize the test bodies so we
can more easily test any git backend in any combination of shallow or
deep fetches.

This is part of #13285.

### How should we test and review this PR?

### Additional information

crates/cargo-test-support/src/lib.rs

@@ -1524,6 +1524,12 @@ impl ArgLineCommandExt for &mut ProcessBuilder {
     }
 }
 
+impl ArgLineCommandExt for &mut Execs {
+    fn arg<S: AsRef<std::ffi::OsStr>>(self, s: S) -> Self {
+        self.arg(s)
+    }
+}
+
 impl ArgLineCommandExt for snapbox::cmd::Command {
     fn arg<S: AsRef<std::ffi::OsStr>>(self, s: S) -> Self {
         self.arg(s)