bug#21634: text-scale-adjust suggestion

[Juri Linkov]

> I think implementing text-scale-adjust on top of repeat-mode would be
> good -- and it could just enable repeat-mode for itself temporarily, I
> think?

repeat-mode = post-command-hook + set-transient-map

We don't need post-command-hook here - this leaves us with just
set-transient-map.  As was already suggested, we need either
to add a new optional arg TIMEOUT to set-transient-map,
or allow a number of seconds in the existing arg KEEP-PRED.

Then another question is how the users could customize the timeout.
One variable to customize timeout for all commands that use
set-transient-map?  Or separate variables for every command:
one for indent-rigidly, one for text-scale-adjust?