Revert "Use correct filename in auto commands."

That commit prevented gitgutter correctly activating on its first run.

This reverts commit 25d428c3900d8f45d2b3f771a734033fb0104262.
Simplify tradeoff between accuracy and speed.

If the plugin slows down your Vim too much, you can have it run less
often by setting `g:gitgutter_eager = 0`.

This replaces the former options `g:gitgutter_on_bufenter` and
`g:gitgutter_on_focusgained`.
Clarify that plugin works with saved buffers.
Clarify FAQ.
Add default mappings.
Prevent error message from showing when s:dummy_sign_id is not set
Merge remote-tracking branch 'upstream/master'