Without this change, any failed assertion in the second (or a later) test
inside a test suite has a chance of double deleting memory, resulting in
a heap corruption. See #1096 for details.
This leaves alone the test cases where we "just" use cl_git_sandbox_init()
and cl_git_sandbox_cleanup(). These methods already take good care to not
double delete a repository.
Fixes#1096
This fix complements cb0ce16bbe and cover the following additional use cases
- retrieving an object which has been previously searched, found and cached
- retrieving an object through an non ambiguous abbreviated id