[pre-commit] Avoid tabs in .pre-commit-config.yaml

When editing .pre-commit-config.yaml with emacs, I tend to get tab-indented
lines, which causes:
...
$ git commit -a
An error has occurred: InvalidConfigError:
==> File .pre-commit-config.yaml
=====> while scanning for the next token
found character that cannot start any token
  in "<unicode string>", line 150, column 1
...

Fix this by adding a "Local Variables" section.

Approved-By: Tom Tromey <tom@tromey.com>