Description of problem:
If a mock root is cached, any changs to the cfg file that would change the
composition of that root file are ignored.  For example, if you change repo
configuration, the next run using autocache will ignore those changes and use
whatever is in the root cache.

Specifically, I tested this by changing:
config_opts['chroot_setup_cmd'] = 'groupinstall build'
to
config_opts['chroot_setup_cmd'] = 'groupinstall build build-minimal'

New files that should have been in the root next time I ran with --autocache
were not there until I rebuilt the root cache.

Version-Release number of selected component (if applicable):
mock-0.6.11-1.fc6

How reproducible:
Always

Steps to Reproduce:
1. run mock with --autocache, cache is created
2. change a cfg file used to build with, change something that would affect root
3. build with new cfg file (and --autocache), changes are not in affect as old
root is cached
  
Expected results:
automatically rebuild root cache when cfg file is newer than root cache, or at
least maybe an --ignorestale option.