fix(workspace): reload current manifest path member only (#15633)

### What does this PR try to resolve?

As of #15625, the manifest path argument in `cargo clippy
--manifest-path foo/Cargo.toml --fix` will be ignored. All the workspace
members will be built. The cause is due to the `reload` usage in
`cargo::ops::fix`. We reload the `root_manifest` in the function, which
contains all workspace members.

Will close #15625.

### How to test and review this PR?

The first commit in the PR demonstrates the current problem, and the
second commit corrects it. Use `cargo test --test testsuite
workspaces::fix_only_check_manifest_path_member` to see the test
results.

Author: weihanglo

src/cargo/core/workspace.rs

@@ -306,7 +306,7 @@ impl<'gctx> Workspace<'gctx> {
     /// This is useful if the workspace has been updated, such as with `cargo
     /// fix` modifying the `Cargo.toml` file.
     pub fn reload(&self, gctx: &'gctx GlobalContext) -> CargoResult<Workspace<'gctx>> {
-        let mut ws = Workspace::new(self.root_manifest(), gctx)?;
+        let mut ws = Workspace::new(&self.current_manifest, gctx)?;
         ws.set_resolve_honors_rust_version(Some(self.resolve_honors_rust_version));
         ws.set_resolve_feature_unification(self.resolve_feature_unification);
         ws.set_requested_lockfile_path(self.requested_lockfile_path.clone());

tests/testsuite/workspaces.rs

@@ -2716,3 +2716,46 @@ fn nonexistence_package_together_with_workspace() {
 "#]])
         .run();
 }
+
+// A failing case from <https://github.com/rust-lang/cargo/issues/15625>
+#[cargo_test]
+fn fix_only_check_manifest_path_member() {
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+            [workspace]
+            members = ["foo", "bar"]
+            resolver = "3"
+            "#,
+        )
+        .file(
+            "foo/Cargo.toml",
+            r#"
+            [package]
+            name = "foo"
+            version = "0.1.0"
+            edition = "2021"
+            "#,
+        )
+        .file("foo/src/main.rs", "fn main() {}")
+        .file(
+            "bar/Cargo.toml",
+            r#"
+            [package]
+            name = "bar"
+            version = "0.1.0"
+            edition = "2021"
+            "#,
+        )
+        .file("bar/src/main.rs", "fn main() {}")
+        .build();
+
+    p.cargo("fix --manifest-path foo/Cargo.toml --allow-no-vcs")
+        .with_stderr_data(str![[r#"
+[CHECKING] foo v0.1.0 ([ROOT]/foo/foo)
+[FINISHED] `dev` profile [unoptimized + debuginfo] target(s) in [ELAPSED]s
+
+"#]])
+        .run();
+}