[stagingView] Fix selection after showing view initially
How to reproduce:
1. Close the view
2. Select a file in a project connected to a repo
3. Show the staging view
Expected: The view is linked with the repo from step 2
Actual: It shows "No Repository Selected"
The problem was that getActivePart in the old code returned null in the
above case. Going via the active workbench window didn't change that.
What works is getting it via execution context.
Bug: 417310
Change-Id: Ib27b9683e52b62f885cb310ce9f177c021cafe94
Signed-off-by: Robin Stocker <robin@nibor.org>