[Feature]: have a mode where the diff view is between the head and the branch point off of main

[imsherrill]

Before submitting

• I searched existing issues and did not find a duplicate.
• I am describing a concrete problem or use case, not just a vague idea.

Area

apps/web

Problem or use case

sometimes you use the same thread/context for a multiple PR workstream. it'd be nice to see only the diffs you are considering in this specific changeset (ignoring diffs in previous prs that have been merged already)

Proposed solution

a new mode that just slightly alters how the diff view works

Why this matters

This would allow a more flexible way to use the product (not one thread per pr workstream)

Smallest useful scope

just a toggle that changes the diff mode somehow

Alternatives considered

No response

Risks or tradeoffs

No response

Examples or references

No response

Contribution

• I would be open to helping implement this.