bug#16438: 24.3.50; `C-x TAB right right' fails with error (mark-inactive)

[Juri Linkov <juri@jurta.org>]

> - But should the region be highlighted *during* C-x TAB?

When the user selects a region, this indicates the intention
to run the next command on the selected region.  So when
a command is going to operate on the region the user should have
the clear indication beforehand to avoid surprises.

Highlighting the region in transient indentation mode is
especially important since this is a new feature,
and without highlighting an unsuspecting user don't know that
a subsequent `right' will operate on the previously selected region.

For the same reason I think the exit condition should not
deactivate the active region because the highlighting
indicates that the next command is operating on the region.

The same problem with exiting the transient indentation mode
happens when the user needs to type the `right' key to move point
without indentation.  Then the user has to exit the sub-mode
explicitly by typing e.g. C-g.  It will also deselect the region.