Define and implement behaviour for symlinks

[jotaen]

For my own purposes, I rely on snapdiff’s default behaviour, which is to skip symlinks. Therefore, I’m actually not sure how well things go in case they are enabled. (Think: circular references.)

It probably doesn’t make sense to resolve symlinks, and count the bytes of the target file. (Also note, the symlink could point to a file outside of the snapshot directory, in which case we certainly don’t want to count these bytes.) But it still might be of interest to see whether a symlink has changed it’s target, or whether it still points to the same file.

I’m wondering whether it’s reasonable enough to include symlinks in the regular file count, without incrementing the byte count, though. So, e.g., a symlink target was changed, increment the “modified” file counter, but not the “modified” byte size.

Otherwise, it might be necessary to introduce a completely separate category or counting logic for symlinks.

(Originally from https://github.com/jotaen/snapdiff/issues/2.)