merge_files: add extra sanity check that input files are in fact files

Now that there is no '-f _' filter in the input loop, merge_files may
encounter a candidate or incumbent filename which resolves to a non-file
object on the filesystem.  If --skip-hash is not used, this sanity check
will be applied during file hashing; however, if --skip-hash is used,
the sanity check will not be applied until the files are compared.