Previously, we've only marked restored buffers as dirty. This PR changes
that behavior in case the buffer has been associated with a file and
that file has changed on disk since the last time Zed stored its
contents.
Example timeline:
1. User edits file in Zed, buffer is dirty
2. User quites Zed with `cmd-q`
3. User changes file on disk: `echo foobar >> file.txt` or `git checkout
file.txt`
4. User starts Zed
5. File/buffer are now marked as having a conflict (yellow icon)
Release Notes:
- Unsaved files that are restored when Zed starts are now marked as
having a conflict if they have been changed on disk since the last time
they were stored.
Demo:
https://github.com/user-attachments/assets/6098b485-b325-49b7-b694-fd2fc60cce64
Thorsten Ball
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