[DRAFT] Address UX issues on Compare revisions feature

Problem

Context !183565 (comment 2398932508)

In the current Compare revisions feature, users can compare:

• branch to a branch
• tag to a branch
• commit to a branch
• commit to a commit

However, these limitations are not surfaced to users well in the UI. Core issues in the UI:

• No visual indication users can search via commit
  • The dropdown placeholder text is Select branch/tag with no mention of commits
  • When users do search by commit, then expand the dropdown, the currently selected commit doesn't even show up in the list
• No visual indication of comparison restrictions
  • Users can currently compare a branch to a branch, or a tag to a branch, or a commit to a branch, commit to a commit. There's literally no way for users to know this. I had to dig through logs of comments just to find an old comment to understand the feature limitations.
• Documentation is poor
  • The Learn more about comparing revisions link on the Compare revisions page actually points to the documentation for Compare branches, which can be misleading / not holistic.
  • Documentation of the feature will be addressed in #525191 (closed).
• Users may want to compare tag to a tag

  • #384887

    

• Expose UI element to allow users to download a patch #525192 (comment 2553248979)
• Consider if we should allow users to add a comment on a Compare line. #15546, related #404540

Proposal

TBD @alyssatrinh