Rename class Diff => TreeDiff
Add git diff.
cleanup.
Add diff api to scm.
More work on git diff.
remove debug string.
Merge branch 'master' into treeDiff

Conflicts:
	ide/app/lib/git/commands/diff.dart
Merge branch 'master' into git_diff2
comments.
Merge pull request #2397 from dart-lang/gaurave-add_git_diff

Add Git Diff command