Add support for forking single branch

[dsseng]

Fixes #25117

Add UI for choosing branch to fork

Change default branch on single-branch forks

[dsseng]

Extra idea: maybe we should support the same option for migrations? It's maybe even more useful in such case when people migrate (essentially fork a repo to their instance) for development purpose

[yp05327]

I understand the point of fork arbitrary branch, but list all branches at one time may cause performance problem.
Loading branches page by page is better, but I'm not sure whether we have this feature. 🤔

[dsseng]

I understand the point of fork arbitrary branch, but list all branches at one time may cause performance problem. Loading branches page by page is better, but I'm not sure whether we have this feature. thinking

Other places where branch list is used do render it in once. Listing branches and putting them into a template shouldn't hog the performance a lot

[denyskon]

@sh7dm Could you please resolve conflicts? I totally missed this PR, but it looks uncomplicated to review. Unfortunately we're already in feature freeze for 1.21 though...

[denyskon]

I'd like for you to use the new translation structure (see my suggestions), but in general, this PR seems to look good

[lunny]

Could you paste a screenshot?

[dsseng]

[delvh]

Alternative design idea:
Offer a radio group with the following possibilities:
- all
- default branch
- custom
Where custom opens a checkbox group of all available branches.
I think this improves both the functionality as well as being more usable, as the two most often used values are easy to find/discern.

[lunny]

Alternative design idea: Offer a radio group with the following possibilities: - all - default branch - custom Where custom opens a checkbox group of all available branches. I think this improves both the functionality as well as being more usable, as the two most often used values are easy to find/discern.

I think it's good idea but could be another PR. At least, this PR allow we fork with a single branch.