Emacs will try to execute .dir-locals.el whenever loading a file under
the subzero directory.  It sets local variables depending on the mode.

Set the fill-column to 80 for c++-mode, c-mode, and python-mode.  The
main use is when using M-q to reformat multi-line comments.

Disable tabs (use spaces instead) in c++-mode, c-mode, and
python-mode.

Set the tab-width to 2 spaces in python-mode.  (The tab-width doesn't
really matter in c++-mode or c-mode thanks to clang-format.)

BUG= none
R=kschimpf@google.com

Review URL: https://codereview.chromium.org/1199133005