v0.18.3-0 QA Review #1644

QA Review to prepare for the v0.18.3 release. Includes backported pull requests listed in #1643.

Checklist

• Proxy items need to be more resilient to unexpected properties #1639: Proxy items need to be more resilient to unexpected properties
• Use my fork of atom-mocha-test-runner #1637: Use my fork of atom-mocha-test-runner
• Undo an initial commit #1635: Undo an initial commit
• Don't attempt to stage or unstage with no selection #1634: Don't attempt to stage or unstage with no selection
• Don't detect executable mode changes on added or deleted files #1633: Don't detect executable mode changes on added or deleted files
• Detached DOM nodes have no parents #1632: Detached DOM nodes have no parents
• Prevent race conditions in focus management code #1631: Prevent race conditions in focus management code (0.18-releases)
• Activate the git tab on item activation #1629: Activate the git tab on item activation
• 🐛 Fix default dir for git repo init #1627: 🐛 Fix default dir for git repo init
• ✅ from CI on Update atom/github on Beta atom#17860.