Git checkout operation notification

[dkuleshov]

User story

As a Che user I want to be informed if git project that I'm working on in Che IDE is modified by git checkout operation irrespective of source of modification.

Scenario

• User A opens a git project P in workspace W in Che IDE
• User A or user B ssh to workspace W
• User A or user B perform Git checkout operation
  • Checking out of a branch
  • Checking out of a commit
  • Checking out of a tag
• Git operation is performed outside of Che by
  • Third party Git client
  • CLI
• User A is notified by Che that Git operation is performed
• All relative data is updated automatically (no refresh needed)

[JamesDrummond]

@vparfonov Provide status label.