Improve UI to ease PR creation with arbitrary other forks

[hawicz]

Feature Description

When creating a PR the "merge into" and "pull from" dropdown boxes show only branches in parent-child related repositories.
Those dropdown boxes also provide a text box where a repo and branch can be manually entered, but it does not currently accept other forks. Please allow it to do so for other forks, including sibling fork or even those that might seem "unrelated" according to the fork network.

There is a workaround where you can edit the url and change it so be e.g.
https://my-gitea-server/targetowner/target-fork/compare/master...otherowner/source-fork:master

Ideally it would also auto-complete, perhaps with a "Show all forks and branches" checkbox to avoid the extra overhead for the common case, but at the very least I should be able to type in "otherowner/source-fork:master", hit enter, and have it actually work.

Screenshots

No response

[lunny]

I have created #32786 which is related. But I'm afraid the unrelated repositories will not be compared anymore after that is merged.