editor: Add diff statistics to file comparison toolbar

[zambetti]

What are you proposing?

A diff statistics indicator +A -D in the buffer toolbar that appears when a single-buffer diff comparison tab is active. This proposal is analogous to the recent PR #60464, but for untracked files.

What becomes easier or possible?

Currently, when comparing untracked files, there is nowhere to look for a summary of the changes; you have to manually scroll to see how many lines were added or removed, or if there are no changes.

Screenshot and Demo

https://github.com/zambetti/zed/tree/diff-stats adds support for this feature and a boolean toolbar.diff_stats in settings.json