Hi,

High CPU consumption after enabling the "auto-revert" for a buffer.
Here is a way to reproduce the issue:
1. on one terminal run command "strace -f -o /tmp/a.log vi -nw"
2. on second terminal, start another emacs and open the file
/tmp/a.log, enable the "auto-revert" mode on the /tmp/a.log buffer.

Typing on 1st terminal(vi), the strace will continually write the
/tmp/a.log, and the emacs try to revert the /tmp/a.log buffer again
and again, then CPU loading turns high.

The function `auto-revert-handler` may be called twice for 2.5 seconds
intervals on a rapidly changed buffer/file.

The root cause is `auto-revert--end-lockout` will call
`auto-revert-handler` in which the `auto-revert--lockout-timer` was
cleared, then the next call `auto-revert-notify-handler` will revert
the buffer immediately regardless the buffer actually was revert just
before.

This patch will record when the buffer was reverted and avoid
reverting it again in the same second.