git: Add support for opening git worktrees (#20164)

This adds support for [git
worktrees](https://matklad.github.io/2024/07/25/git-worktrees.html). It
fixes the errors that show up (git blame not working) and actually adds
support for detecting git changes in a `.git` folder that's outside of
our path (and not even in the ancestor chain of our root path).

(While working on this we discovered that our `.gitignore` handling is
not 100% correct. For example: we do stop processing `.gitignore` files
once we found a `.git` repository and don't go further up the ancestors,
which is correct, but then we also don't take into account the
`excludesFile` that a user might have configured, see:
https://git-scm.com/docs/gitignore)


Closes https://github.com/zed-industries/zed/issues/19842
Closes https://github.com/zed-industries/zed/issues/4670

Release Notes:

- Added support for git worktrees. Zed can now open git worktrees and
the git status in them is correctly handled.

---------

Co-authored-by: Antonio <antonio@zed.dev>
Co-authored-by: Bennet <bennet@zed.dev>

crates/search/src/project_search.rs

@@ -2024,6 +2024,7 @@ pub fn perform_project_search(
     text: impl Into<std::sync::Arc<str>>,
     cx: &mut gpui::VisualTestContext,
 ) {
+    cx.run_until_parked();
     search_view.update(cx, |search_view, cx| {
         search_view
             .query_editor