[FIXED JENKINS-13202] Don't set mtime or mode on symlinks

Previously, the untar code tries to set the last modified time and mode
on every untarred file.  However, if the tar contains a broken symlink,
or a symlink that points to a file that has not been untarred yet, the
time/mode setting would fail on the broken symlink.

Symlinks don't have meaningful modified times or modes of their own, so
only set these values on non-symlinks.

Rename the file "a" in the test to expose the bug.