Git: Support optional branch picking in the repository picker

[egamma]

No description provided.

[joaomoreno]

Pushing this out, unfortunately.

[joaomoreno]

A RemoteSourceProvider can now optionally implemented getBranches:

export interface RemoteSourceProvider {
	getBranches?(url: string): ProviderResult<string[]>;
}

I've implemented it for our Github extension.

And when passing branch: true to the git.api.getRemoteSources API command, the user will be prompted to pick a branch, if the provider implements getBranches:

await commands.executeCommand('git.api.getRemoteSources', {
	providerLabel: (provider: RemoteSourceProvider) => `Test this for ${provider.name}`,
	urlLabel: `Test this for URL`,
	branch: true
});

And the result will be the following object instead of a string:

export interface PickRemoteSourceResult {
	readonly url: string;
	readonly branch?: string;
}