Improve regexp to prevent false positives

If a filename happened to contain "db" and enough "rwx" characters before, then
this test would previously fail. For example:

```
drwxr-xr-x gitlab-runner/gitlab-runner      0 2015-04-02 07:46 uploads/tmp/cassidy.stokes8477/gitlabhq/36d972fa55d6b44810fc6fd843473adb/
```

Adding a space before the "db" match string tightens up the regexp and reduces the
chance of an unintended match.

See merge request !489