Currently, if the `certs/` directory has a symlink inside, we skip it.
That is not really intended, it's an unfortunate side-effect of skipping
regular files.
To fix this, this patch adjusts the logic to only ignore regular files
instead. It also adds a message when a directory is skipped, to make it
easier to debug permission issues.
Thanks to @erjoalgo for reporting this in
https://github.com/albertito/chasquid/pull/39, and providing an
alternative patch!
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