Issue title: Cannot stage merged changes
 Issue body: ### Description
During a rebase I had to merge changes. I cannot stage those changes using the SCM view (clicking the `+` button has no effect).

### Reproduction Steps
1. Rebase a branch so that changes need to be merged manually
2. Merge those changes
3. Try to stage those changes

**OS and Theia version:**
tested in gitpod.io

**Diagnostics:** 
there is no error on the console
 Comments: 
 Comment 0: In order to reproduce merge conflict: https://stackoverflow.com/questions/43702944/reproducing-git-merge-conflict-dd
{'source': 'GithubIssues', 'repo_name_with_owner': 'theia-ide/theia', 'issue_id': 455809158, 'repo_stars': 5279.0}