The "Open this repository on GitLab button" is always trying to open the file in the "master" branch

[beenje]

Description

The "master" branch is hard-coded when using the "Open this repository on GitLab" button:

jupyterlab-gitlab/src/browser.ts

Lines 53 to 60 in ce6f375

	if (resource.repository) {
	url = URLExt.join(
	url,
	resource.repository,
	'tree',
	'master',
	resource.path
	);

The button should use the repository default branch instead.

Note that when browsing a repository, the default branch is now retrieved via the API (see dd985a3) but this isn't used for the toolbar button yet.