Merge branch 'gp/status-rebase-i-info'

Teach "git status" to show a more detailed information regarding
the "rebase -i" session in progress.

* gp/status-rebase-i-info:
  status: add new tests for status during rebase -i
  status: give more information during rebase -i
  status: differentiate interactive from non-interactive rebases
  status: factor two rebase-related messages together