simplify scm/git data flow

[akosyakov]

ScmService should be a single source of the whole scm state, git extension should populate it and provide git specific view instead of duplicating this state partially and trying to sync it.

For right now repositories state is duplicated several times:

• GitContribution.dirtyResporitories duplicating GitRepositoryProvider.allRepositories which duplicating ScmService.repositories
• GitRepositoryProvider.selectedRepository duplicating ScmService.selectedRepository

GitRepositoryProvider should be a git specific facade to ScmService. Handling of ScmProvider should be moved to GitRepositoryProvider instead of keeping allRepositories and GitContribution.dirtyResporitories.

[akosyakov]

Also state of changes is duplicated in /~https://github.com/theia-ide/theia/blob/93fb9e74d9e807b539222fdc67d6b5a6663251f7/packages/git/src/browser/git-contribution.ts#L330 and in /~https://github.com/theia-ide/theia/blob/93fb9e74d9e807b539222fdc67d6b5a6663251f7/packages/git/src/browser/git-commands.ts#L76-L120

It should be computed only once before rendering and then rendering and git commands should work on exactly the same state.

Right now it could happen that SCM widget shows data for one state, but commands are performed on other.