Better handling of Untracked files.
Add git add command.
Add git add option to UI.
Add git add api to scm.dart.
Show git add only for scm projects.
Fix bug in updating git index.
Skip git index update for untracked files.
Refresh git status after git add.
Addressed comments (nits).
Show git add only when valid.
Merge pull request #1825 from dart-lang/git_add

Git add