Allow Changes totals to focus on the current PR or committed branch

[CadenEl]

I use Changes to review my current branch or PR, but uncommitted files can swamp its total. In one example, a branch with 4 committed lines showed +10,008 because Changes also counted 2 staged lines, 2 unstaged lines, and a 10,000-line untracked report. That total is correct for the working tree. It just doesn't tell me how big the committed change is.

Proposal

Add an optional Changes comparison setting under Settings → Diffs & pull requests:

• All changes, including local work stays the default and keeps today's behavior.
• Current PR or committed branch makes Changes show only the checked-out branch's open PR, or its committed diff when there is no open PR. Staged, unstaged, and untracked work stays available under Uncommitted.

This changes what Changes compares, not how it counts. Related: #30089 and #19530.

To reproduce

On a feature branch from main, commit a few lines, stage a few more, leave a few unstaged, and add a large untracked file. Open the branch's Changes row.

Recorded example

I built the option on main at 6577c6b618 and recorded it in an isolated web preview. The repository and conversation are synthetic, but the totals come from real Git reads.

Default: +10,008	Current PR or committed branch: +4

More screenshots: the setting · local work under Uncommitted

Recording of the switch (10 seconds, no audio):

changes-comparison.mp4

[CadenEl]

I opened draft PR #32228 with the optional setting, before/after screenshots, and a short recording.