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.
Also, we don't `git status` against entire git worktree/index anymore.

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

src/cargo/ops/cargo_package/vcs.rs

@@ -1,6 +1,7 @@
 //! Helpers to gather the VCS information for `cargo package`.
 
 use std::collections::HashSet;
+use std::path::Path;
 use std::path::PathBuf;
 
 use anyhow::Context as _;
@@ -173,7 +174,9 @@ fn git(
     // - 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)?;
@@ -263,12 +266,18 @@ fn dirty_files_outside_pkg_root(
 }
 
 /// Helper to collect dirty statuses for a single repo.
-fn collect_statuses(repo: &git2::Repository, dirty_files: &mut Vec<PathBuf>) -> CargoResult<()> {
+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);
@@ -300,8 +309,16 @@ fn status_submodules(repo: &git2::Repository, dirty_files: &mut Vec<PathBuf>) ->
         // 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(())
 }
+
+/// 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('\\', "/")
+}