Add Git::checkoutHeadForEditor

This moves the logic from Editor::checkoutHead
Use same dialog language as GitHub for Mac
revert -> checkoutHead
Move checkout head editor specs to git spec
:lipstick:
Remove spec now covered in git spec
Add since the last Git commit to detailed message
Make message map to command name
confirmCheckoutHead -> confirmCheckoutHeadRevision
Merge pull request #3264 from atom/ks-move-checkout-head-to-git-class

Move checkout head to Git class
Upgrade to apm 0.91
:memo: Mention --build-dir and --install-dir

Closes #3220
:memo: :lipstick:
:memo: Use script/build in --build-dir example
Upgrade to autosave@0.15
Update to deprecation-cop@0.9
Upgrade to incompatible-packages@0.8
Use opacity to blink cursor instead of visibility so blink can be styled
Prepare 0.123
:memo: Bool -> Boolean