Issue #355: Infinite loop in find_repository_root() with missing files (blais/beancount)

[Martin Michlmayr]

New issue 355: Infinite loop in find_repository_root() with missing files
https://bitbucket.org/blais/beancount/issues/355/infinite-loop-in-find_repository_root-with

Martin Michlmayr:

find_repository_root() in beancount/utils/test_utils.py does:

```
while not all(path.exists(path.join(filename, sigfile))
for sigfile in ('PKGINFO', 'COPYING', 'README')):
filename = path.dirname(filename)
```

This ends up in an infinite loop looking at `/` if for some reason someone accidentally
removed one of these 3 files.