perf(cargo-package): match certain path prefix with pathspec

`check_repo_state` checks the entire git repo status.
This is usually fine if you have only a few packages in a workspace.

For huge monorepos, it may hit performance issues.

For example,
on awslabs/aws-sdk-rust@2cbd34d
the workspace has roughly 434 members to publish.
`git ls-files` reported us 204379 files in this Git repository.
That means git may need to check status of all files 434 times.
That would be `204379 * 434 = 88,700,486` checks!

Moreover, the current algorithm is finding the intersection of
`PathSource::list_files` and `git status`.
It is an `O(n^2)` check.
Let's assume files are evenly distributed into each package,
so roughly 470 files per package.
If we're unlucky to have some dirty files, say 100 files.
We will have to do `470 * 100 = 47,000` times of path comparisons.

Even worse, because we `git status` everything in the repo,
we'll have to it for all members,
even when those dirty files are not part of the current package in question.
So it becomes `470 * 100 * 434 = 20,398,000`!

Instead of comparing with the status of the entire repository,
this patch use the magic pathspec[1] to tell git only reports
paths that match a certain path prefix.

This wouldn't help the `O(n^2)` algorithm,
but at least it won't check dirty files outside the current package.

[1]: https://git-scm.com/docs/gitglossary#Documentation/gitglossary.txt-aiddefpathspecapathspec

src/cargo/ops/cargo_package.rs

@@ -815,7 +815,8 @@ fn check_repo_state(
         // - ignored (in case the user has an `include` directive that
         //   conflicts with .gitignore).
         let mut dirty_files = Vec::new();
-        collect_statuses(repo, &mut dirty_files)?;
+        let pathspec = relative_pathspec(repo, pkg.root());
+        collect_statuses(repo, &[pathspec.as_str()], &mut dirty_files)?;
         // Include each submodule so that the error message can provide
         // specifically *which* files in a submodule are modified.
         status_submodules(repo, &mut dirty_files)?;
@@ -857,16 +858,27 @@ fn check_repo_state(
         }
     }
 
+    /// Use pathspec so git only matches a certain path prefix
+    fn relative_pathspec(repo: &git2::Repository, pkg_root: &Path) -> String {
+        let workdir = repo.workdir().unwrap();
+        let relpath = pkg_root.strip_prefix(workdir).unwrap_or(Path::new(""));
+        // to unix separators
+        relpath.to_str().unwrap().replace('\\', "/")
+    }
+
     // Helper to collect dirty statuses for a single repo.
     fn collect_statuses(
         repo: &git2::Repository,
+        pathspecs: &[&str],
         dirty_files: &mut Vec<PathBuf>,
     ) -> CargoResult<()> {
         let mut status_opts = git2::StatusOptions::new();
         // Exclude submodules, as they are being handled manually by recursing
         // into each one so that details about specific files can be
         // retrieved.
-        status_opts
+        pathspecs
+            .iter()
+            .fold(&mut status_opts, git2::StatusOptions::pathspec)
             .exclude_submodules(true)
             .include_ignored(true)
             .include_untracked(true);
@@ -901,7 +913,7 @@ fn check_repo_state(
             // If its files are required, then the verification step should fail.
             if let Ok(sub_repo) = submodule.open() {
                 status_submodules(&sub_repo, dirty_files)?;
-                collect_statuses(&sub_repo, dirty_files)?;
+                collect_statuses(&sub_repo, &[], dirty_files)?;
             }
         }
         Ok(())