Draft: feat(view issues-and-mrs): checkout local branch by merge request

This is a consequence of this merge request.

How it work

What i did

1. commit For data providers, when initializing tree items, I wrote the name of their class into the context value (see src/data_providers/current_branch.ts and src/data_providers/issuable.ts). This allows you to write conditions for view items in the manifest.

...
"view/item/context": [
  {
    "command": "gl.commandName",
    "when": "view =~ /viewInternalId/ && viewItem == ItemClassName",
    "group": "someGroup"
  }
],
...

2. commit01, commit02 Changed GitExtensionWrapper structure to singleton because this class does not need instances

3. commit added method for get git repositories by workspace object.

4. commit Added shorthand version of merge request type from restissuable type.

5. commit01, commit02 Added method that fetches and checkout only the local branch by the merge request object

Related to #63 (closed)