Web: Add autocompletion to repository compare page
Created by: umpox
This PR:
- Adds autocomplete to repo compare page
- Updates the landing area when first loading the compare page
- Enables speculative search to support additional git revspec syntax
- Updates other copy and UI
Part of https://github.com/sourcegraph/sourcegraph/issues/22677
https://user-images.githubusercontent.com/9516420/129737519-7988c8b5-adc2-4173-8c39-667b22263e0b.mov