fix(package): detect dirty files when run from workspace member

[weihanglo]

What does this PR try to resolve?

Fixes #16478

When running cargo package from a workspace member directory,
the VCS dirty check fails to detect untracked files due to the
pathspec being interpreted relative to the current working directory
instead of the repository root.

Use :(top) magic signature to make the pathspec relative to the repo root,
not the current working directory.

See also https://git-scm.com/docs/gitglossary#Documentation/gitglossary.txt-aiddefpathspecapathspec

How to test and review this PR?

[rustbot]

r? @epage

rustbot has assigned @epage.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[epage]

CC @Byron

[epage]

Not the biggest expert on this pathspec stuff but seems to work?

View changes since this review

[weihanglo]

We already have that it file listing btw:

cargo/src/cargo/sources/path.rs

Lines 720 to 730 in 1436cf9

	let pathspec = {
	// Include the package root.
	let mut include = BString::from(":(top)");
	include.push_str(package_prefix.as_ref());
	// Exclude the target directory.
	let mut exclude = BString::from(":!(exclude,top)");
	exclude.push_str(target_prefix.as_ref());
	vec![include, exclude]
	};