Description of problem:
The latest upgrade of mock on epel-testing broke usage of mock on el6


Version-Release number of selected component (if applicable):
mock-1.3.4-1.el6.noarch

How reproducible:
Deterministic

Steps to Reproduce:
1. curl -O https://kojipkgs.fedoraproject.org//packages/mock/1.3.4/1.el6/src/mock-1.3.4-1.el6.src.rpm
2. mock --root epel-6-x86_64 --rebuild mock-1.3.4-1.el6.src.rpm --resultdir .

Actual results:
INFO: mock.py version 1.3.4 starting (python version = 2.6.6)...
Start: init plugins
INFO: tmpfs initialized
INFO: selinux disabled
Finish: init plugins
Start: run
INFO: Start(mock-1.3.4-1.el6.src.rpm)  Config(epel-6-x86_64)
Start: clean chroot
Finish: clean chroot
Start: chroot init
INFO: mounting tmpfs at /var/lib/mock/epel-6-x86_64/root.
INFO: calling preinit hooks
INFO: enabled root cache
Start: unpacking root cache
Finish: unpacking root cache
INFO: enabled yum cache
Start: cleaning yum metadata
Finish: cleaning yum metadata
INFO: enabled HW Info plugin
ERROR: Exception(mock-1.3.4-1.el6.src.rpm) Config(epel-6-x86_64) 0 minutes 5 seconds
INFO: Results and/or logs in: .
INFO: Cleaning up build root ('cleanup_on_failure=True')
Start: clean chroot
INFO: unmounting tmpfs.
Finish: clean chroot
Traceback (most recent call last):
  File "/usr/libexec/mock/mock", line 886, in <module>
    main()
  File "/usr/lib/python2.6/site-packages/mockbuild/trace_decorator.py", line 89, in trace
    result = func(*args, **kw)
  File "/usr/libexec/mock/mock", line 701, in main
    run_command(options, args, config_opts, commands, buildroot, state)
  File "/usr/lib/python2.6/site-packages/mockbuild/trace_decorator.py", line 89, in trace
    result = func(*args, **kw)
  File "/usr/libexec/mock/mock", line 783, in run_command
    do_rebuild(config_opts, commands, buildroot, args)
  File "/usr/lib/python2.6/site-packages/mockbuild/trace_decorator.py", line 89, in trace
    result = func(*args, **kw)
  File "/usr/libexec/mock/mock", line 520, in do_rebuild
    post=post_build, clean=clean)
  File "/usr/lib/python2.6/site-packages/mockbuild/trace_decorator.py", line 89, in trace
    result = func(*args, **kw)
  File "/usr/libexec/mock/mock", line 464, in rebuild_generic
    commands.init(prebuild=not config_opts.get('short_circuit'))
  File "/usr/lib/python2.6/site-packages/mockbuild/trace_decorator.py", line 89, in trace
    result = func(*args, **kw)
  File "/usr/lib/python2.6/site-packages/mockbuild/backend.py", line 125, in init
    self.buildroot.initialize(**kwargs)
  File "/usr/lib/python2.6/site-packages/mockbuild/trace_decorator.py", line 89, in trace
    result = func(*args, **kw)
  File "/usr/lib/python2.6/site-packages/mockbuild/buildroot.py", line 90, in initialize
    self._init(prebuild=prebuild, do_log=do_log)
  File "/usr/lib/python2.6/site-packages/mockbuild/trace_decorator.py", line 89, in trace
    result = func(*args, **kw)
  File "/usr/lib/python2.6/site-packages/mockbuild/buildroot.py", line 123, in _init
    self.plugins.call_hooks('preinit')
  File "/usr/lib/python2.6/site-packages/mockbuild/trace_decorator.py", line 89, in trace
    result = func(*args, **kw)
  File "/usr/lib/python2.6/site-packages/mockbuild/plugin.py", line 69, in call_hooks
    hook(*args, **kwargs)  
  File "/usr/lib/python2.6/site-packages/mockbuild/trace_decorator.py", line 89, in trace
    result = func(*args, **kw)
  File "/usr/lib/python2.6/site-packages/mockbuild/plugins/hw_info.py", line 48, in _PreInitHook
    output = mockbuild.util.do(cmd, shell=False, returnOutput=True, raiseExc=False)
  File "/usr/lib/python2.6/site-packages/mockbuild/trace_decorator.py", line 89, in trace
    result = func(*args, **kw)
  File "/usr/lib/python2.6/site-packages/mockbuild/util.py", line 532, in do
    preexec_fn=preexec,
  File "/usr/lib64/python2.6/subprocess.py", line 642, in __init__
    errread, errwrite)
  File "/usr/lib64/python2.6/subprocess.py", line 1238, in _execute_child
    raise child_exception  
OSError: [Errno 2] No such file or directory

Expected results:
Package is rebuild without any problem

Additional info:
sh$ grep -C5 "bin/df" /usr/lib/python2.6/site-packages/mockbuild/plugins/hw_info.py
        cmd = ["/usr/bin/free"]
        output = mockbuild.util.do(cmd, shell=False, returnOutput=True, raiseExc=False)
        out.write("\n\nMemory:\n")
        out.write(output)

        cmd = ["/usr/bin/df", "-H", self.buildroot.make_chroot_path()]
        output = mockbuild.util.do(cmd, shell=False, returnOutput=True, raiseExc=False)
        out.write("\n\nStorage:\n")
        out.write(output)

        out.close()

CentOS6 does not have merged /usr/bin and /bin
sh$ file /usr/bin/df /bin/df
/usr/bin/df: cannot open `/usr/bin/df' (No such file or directory)
/bin/df:     ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked (uses shared libs), for GNU/Linux 2.6.18, stripped