Validate if branch exists before creating project

[benjaoming]

When importing a project using "Import Manually", the branch field can be specified or left empty (to choose the repo's default branch).

Let's say that we specify it like this:

Validation is needed / nice-to-have

If the field is specified with a branch that does NOT exist, the project is again locked* in the same state as #9367 - because the initial checkout fails (the branch does not exist), we will not be able to select a new branch using the Settings UI:

locked*) the workaround is to save the project with a blank default branch. This will trigger a new build to start, using the default branch of the git repository.

Edit: description is updated

[stsewd]

I think re-saving the project should fix this problem, that way the project goes back to choose the default branch.

[benjaoming]

Oh yeah, that's true! And a good, simple work-around. The first build fails correctly since the branch does not exist:

Afterwards, the list of choices in "Default branch" is empty. But clicking "Save" again will trigger the new functionality to correctly use the repo's default branch instead of the broken one 👍

[humitos]

At some point, if we implement this part of #10352

Detection of a missing configuration file without building

We could use the same mechanism to check/detect if the defined branch exists or not.