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