Allow overriding hardcoded branch for git submodules

[nejch]

What would you like Renovate to be able to do?
Add the ability to configure a branch to update git submodules from, instead of the default master, e.g. in cases where you want to update internal dependencies from devel/develop/development etc.

Currently the git submodule manager defaults to a hardcoded master if the super-repo .gitmodules doesn't explicitly specify a branch for the submodule:

renovate/lib/manager/git-submodules/extract.ts

Lines 42 to 48 in 65839b7

	(await Git().raw([
	'config',
	'--file',
	gitModulesPath,
	'--get',
	`submodule.${submoduleName}.branch`,
	])) || 'master'

This would allow updating internal projects using git flow, but in recent days there has been some development that will make this more relevant as GitHub and GitLab (and even discussions for git) are either making the default configurable or moving away from master as the default branch in some cases.

• GitHub https://twitter.com/Una/status/1271180494944829441
• GitLab https://gitlab.com/gitlab-org/gitlab/-/issues/221164, https://gitlab.com/gitlab-org/gitlab/-/issues/221013, etc

Describe the solution you'd like
An option in renovate.json to configure the branch, e.g.:

  "git-submodules": {
    "enabled": true,
    "submoduleBranch": "develop"
  }

Bonus: it'be great if this could be combined with package rules or similar (so that e.g. libs/internal can have development while libs/external have master)

Describe alternatives you've considered
You can define tracking branches explicitly for every project in .gitmodules but this is not always desired and can be a pain for projects with many submodules.

[rarkins]

This may require a refactor of our git-submodules first.

[nejch]

@rarkins I realized there's a much quicker solution that would at least solve the "non-master default branch" use case for this, if not the fully manually configurable in renovate.json. The latter is a much more niche use-case anyway, so I'd be happy to close this if at least the remote default branch can be picked up correctly (instead of hardcoded master). I think this maybe requires just an additional function, no refactoring.

Something similar was already discussed in the initial PR when the submodule feature was added, but not used for some reason:

git ls-remote --symref <URL> HEAD

will print out the default remote branch without cloning, regardless of the platform, e.g.:

$ git ls-remote --symref https://github.com/renovatebot/renovate.git HEAD
ref: refs/heads/master  HEAD
2ac6c392c27c3fd423a3f88d9d9221fb581b2368        HEAD

With some splitting/trimming it should be easy to get the branch from the first line.

So maybe an additional function is enough, which can then be called in getBranch instead of using master (just need to get the URL before the branch):

async function getDefaultBranch(
  subModuleUrl: string
): Promise<string> {
  return (
    (await Git().raw([
      'ls-remote',
      '--symref',
      subModuleUrl,
      'HEAD',
    ]))
  ).trim(); // and some splitting
}

GitHub will switch to main for new projects on October 1st. I'll try to learn just enough TS to take a stab at this by then, but maybe this is just a 15 min job for you so feel free as well. I'll just be happy to see it done regardless :D Thanks for your time!

[github-actions[bot]]

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