add test to run git2 on a gitoxide-fetched repository and vice-versa

We do that on both the crates-io clone as well as on the clone for
a git dependency. The first `cargo` invocation is configured to use
whatever wouldn't be the default, whereas the second one will use
whatever is the default, effectively mixing the implementations.

That way we should assure that users can switch back and forth
between implementations without issues with their local state.

src/cargo/core/features.rs

@@ -790,6 +790,9 @@ pub struct GitoxideFeatures {
     /// Checkout git dependencies using `gitoxide` (submodules are still handled by git2 ATM, and filters
     /// like linefeed conversions are unsupported).
     pub checkout: bool,
+    /// A `gitoxide` feature which doesn't have any meaning except for forcing `always_test_gitoxide` builds
+    /// to not actually enabled `gitoxide`.
+    pub internal_use_git2: bool,
 }
 
 impl GitoxideFeatures {
@@ -799,6 +802,7 @@ impl GitoxideFeatures {
             shallow_index: true,
             checkout: true,
             shallow_deps: true,
+            internal_use_git2: false,
         }
     }
 
@@ -810,6 +814,7 @@ impl GitoxideFeatures {
             shallow_index: false,
             checkout: true,
             shallow_deps: false,
+            internal_use_git2: false,
         }
     }
 }
@@ -823,6 +828,7 @@ fn parse_gitoxide(
         shallow_index,
         checkout,
         shallow_deps,
+        internal_use_git2,
     } = &mut out;
 
     for e in it {
@@ -831,6 +837,7 @@ fn parse_gitoxide(
             "shallow_index" => *shallow_index = true,
             "shallow_deps" => *shallow_deps = true,
             "checkout" => *checkout = true,
+            "internal_use_git2" => *internal_use_git2 = true,
             _ => {
                 bail!("unstable 'gitoxide' only takes `fetch`, 'shallow_index', 'shallow_deps' and 'checkout' as valid inputs")
             }

tests/testsuite/git.rs

@@ -1827,6 +1827,50 @@ fn fetch_downloads() {
     p.cargo("fetch").with_stdout("").run();
 }
 
+#[cargo_test]
+fn fetch_downloads_with_git2_first_then_with_gitoxide_and_vice_versa() {
+    let bar = git::new("bar", |project| {
+        project
+            .file("Cargo.toml", &basic_manifest("bar", "0.5.0"))
+            .file("src/lib.rs", "pub fn bar() -> i32 { 1 }")
+    });
+    let feature_configuration = if cfg!(always_test_gitoxide) {
+        // When we are always using `gitoxide` by default, create the registry with git2 as well as the download…
+        "-Zgitoxide=internal_use_git2"
+    } else {
+        // …otherwise create the registry and the git download with `gitoxide`.
+        "-Zgitoxide=fetch"
+    };
+
+    let p = project()
+        .file(
+            "Cargo.toml",
+            &format!(
+                r#"
+                    [package]
+                    name = "foo"
+                    version = "0.5.0"
+                    authors = []
+                    [dependencies.bar]
+                    git = '{}'
+                "#,
+                bar.url()
+            ),
+        )
+        .file("src/main.rs", "fn main() {}")
+        .build();
+    p.cargo("fetch")
+        .arg(feature_configuration)
+        .masquerade_as_nightly_cargo(&["gitoxide must be available"])
+        .with_stderr(&format!(
+            "[UPDATING] git repository `{url}`",
+            url = bar.url()
+        ))
+        .run();
+
+    p.cargo("fetch").with_stdout("").run();
+}
+
 #[cargo_test]
 fn warnings_in_git_dep() {
     let bar = git::new("bar", |project| {