Description of problem:
Using the --autocache option with mock in F7 generates a python stack trace.  To
my untrained eye, it appears that the yum.conf symlink is unpacked as part of
the autocache, and then mock tries to create it a second time after unpacking
the cached root.
 
[rpmbuild@owl ~]$ time mock -r core7-64 --autocache
/space/src/fedora/vavoom/F-7/vavoom-1.23.1-1.fc7.src.rpm 
init
clean
prep
This may take a while
unpack cache
Traceback (most recent call last):
  File "/usr/bin/mock", line 1046, in <module>
    main()
  File "/usr/bin/mock", line 1043, in main
    do_rebuild(config_opts, srpms)
  File "/usr/bin/mock", line 910, in do_rebuild
    my.prep()
  File "/usr/bin/mock", line 254, in prep
    self._prep_install()
  File "/usr/bin/mock", line 659, in _prep_install
    os.symlink('../yum.conf', os.path.join(yumdir, 'yum.conf'))
OSError: [Errno 17] File exists

real    0m20.154s
user    0m7.986s
sys     0m7.151s

Version-Release number of selected component (if applicable):
mock-0.6.12-1.fc7.x86_64