Concern about new binding.

[Ergus]

Hi: 

In a recent commit I see that the C-x g binding was taken for
revert-buffer.

I suppose this should have been discussed previously, unless I don't
find the thread... but I have some concerns about this.

1) In general `C-x g` is used by magit; I know this is not a priority
(cause magit is an external package), but magit is a very popular
package.

2) In some threads before we have discussed that relative short
combinations must be reserved to maps in order to economize them... 

3) In spite of revert-buffer is a very useful command it is useless in
many conditions like when auto-revert-mode is enable... so maybe it
makes sense to bind it to something longer (ex: C-x g r)??

There are more commands that could be set in a map inside C-x g IMHO:
toggle-read-only
revert-buffer-with-fine-grain
something to enable/disable font-lock
 etc

This way instead of limiting; could open a big spot to set useful
commands. I understand that longer commands are harder to discover, but
hopefully we could have which-key in the next release? 

I hope I am writing this soon enough that the author could consider
revert this change maybe with something more "general" does it makes
sense??

Best,
Ergus.