Enhance commit view compare functionality

[umpox]

Originally from https://github.com/sourcegraph/sourcegraph/issues/22677

The other place it would be helpful to have the compare functionality is when viewing a commit. Users would like to select a given revision from a dropdown to compare to while in this view: https://sourcegraph.com/github.com/sourcegraph/sourcegraph/-/commit/85073d2fa79f0a106948494a0a84ae7c2c859a6a?visible=3

Common request and most recently called out by:

• https://app.hubspot.com/contacts/2762526/company/6286373436
• https://app.hubspot.com/contacts/2762526/company/6288462706
• https://app.hubspot.com/contacts/2762526/company/6330348409

[github-actions]

Heads up @AlicjaSuska @umpox @valerybugakov @5h1rU @pdubroy - the "team/frontend-platform" label was applied to this issue.

[jasongornall]

Bumping for @AlicjaSuska set p3 as lower priority

[jasongornall]

p3 = nice to have but not urgent. can work on when you have free time