apply-manifest does not perform filesystem operations

[dmerejkowsky]

Steps to reproduce:

• Create a manifest file containing a symlink operation
• Run tsrc apply-manifest
• No file system operation is performed.

Note: this is because of this code:

    manifest = tsrc.manifest.load(manifest_path)
    workspace.repos = repos_from_config(manifest, workspace.config)

We override workspace.repos but instead we should do something like:

  workspace.local_manifest = FileManifest(manifest_path)

and override the local_manifest instance of the workspace.

This needs a bit of refactoring, for instance by having an abstract LocalManifest class, which would be implemented by ClonedManifestRepo and ManifestPath or something ....