set git-commit-overlong-summary-face correctly
Fix weirdness when dired loads .dir-locals.el

If one tries to open a directory containing the .dir-locals.el file
with dired, then there apparently isn't a buffer and buffer-file-name
returns nil, causing locate-dominating-file to ralph.

Now the code establishes a "safe buffer file name" and passes that to
locate-dominating-file.