If we have files in partial/ from a previous invocation or similar such
those could be symlinks created by file:// sources. The code is
expecting only real files through and happily changes owner,
modification times and permission on the file the symlink points to
which tend to be files we have no business in touching in this way.
Permissions of symlinks shouldn't be changed, changing owner is usually
pointless to, but just to be sure we pick the easy way out and use
lchown, check for symlinks before chmod/utimes.
Reported-By: Mattia Rizzolo on IRC