fix(package): use relpath to cwd for vcs dirtiness report (#14970)

### What does this PR try to resolve?

Address
#14968 (comment)

> I think the ideal solution is to be relative to the current directory
but that takes more work and this incremental improvement is great!

Sorry I should have noticed your comment earlier.

src/cargo/ops/cargo_package.rs

@@ -796,7 +796,7 @@ fn check_repo_state(
         .and_then(|p| p.to_str())
         .unwrap_or("")
         .replace("\\", "/");
-    let Some(git) = git(src_files, &repo, &opts)? else {
+    let Some(git) = git(gctx, src_files, &repo, &opts)? else {
         // If the git repo lacks essensial field like `sha1`, and since this field exists from the beginning,
         // then don't generate the corresponding file in order to maintain consistency with past behavior.
         return Ok(None);
@@ -805,6 +805,7 @@ fn check_repo_state(
     return Ok(Some(VcsInfo { git, path_in_vcs }));
 
     fn git(
+        gctx: &GlobalContext,
         src_files: &[PathBuf],
         repo: &git2::Repository,
         opts: &PackageOpts<'_>,
@@ -823,12 +824,13 @@ fn check_repo_state(
         // Find the intersection of dirty in git, and the src_files that would
         // be packaged. This is a lazy n^2 check, but seems fine with
         // thousands of files.
-        let workdir = repo.workdir().unwrap();
+        let cwd = gctx.cwd();
         let mut dirty_src_files: Vec<_> = src_files
             .iter()
             .filter(|src_file| dirty_files.iter().any(|path| src_file.starts_with(path)))
             .map(|path| {
-                path.strip_prefix(workdir)
+                pathdiff::diff_paths(path, cwd)
+                    .as_ref()
                     .unwrap_or(path)
                     .display()
                     .to_string()

tests/testsuite/package.rs

@@ -1099,6 +1099,20 @@ Cargo.toml
 
 to proceed despite this and include the uncommitted changes, pass the `--allow-dirty` flag
 
+"#]])
+        .run();
+
+    // cd to `src` and cargo report relative paths.
+    p.cargo("package")
+        .cwd(p.root().join("src"))
+        .with_status(101)
+        .with_stderr_data(str![[r#"
+[ERROR] 1 files in the working directory contain changes that were not yet committed into git:
+
+../Cargo.toml
+
+to proceed despite this and include the uncommitted changes, pass the `--allow-dirty` flag
+
 "#]])
         .run();
 }