MODULES-1596 - Repository repeatedly destroyed/created with force

The `retrieve` method was calling `create` and `destroy` on every run
with `force => true`. Retrieve should not be making any changes to the
system, so removed that code, and updated `working_copy_exists` to make
sure that the directory not only contains a `.git` directory, but also
if `source` is specified it also matches `#{path}/.git/config` so that
it will overwrite a git repo with a different source.

Updated tests to not check for the old broken behavior. Added a regression test.
Merge pull request #225 from mhaskel/MODULES-1596

MODULES-1596 - Repository repeatedly destroyed/created with force