bug#20323: 25.0.50; Emacs's .dir-locals.el should set indent-tabs-mode to nil for emacs-lisp-mode and c-mode

[Glenn Morris]

I've long since done what seems to me to be the sensible thing:

  use a mode-hook to set indent-tabs-mode to whatever the (pre-existing)
  file I'm editing seems to use.

I won't claim it's always right (for those files where the indentation
is mixed), but it's worked pretty well for me.

The fact that indent-tabs-mode has been added to dir-locals now breaks
this, since dir-locals take precedence over mode hooks (bug#1986).

So for me, this change increases the likelihood of me committing stuff
that does not match the existing indentation style. Ie, adding it to
dir-locals without untabifying every file (something which has already
been rejected) has made things worse IMO.