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
It's implemented in revwalk.c so it has access to the revision
walker's commit cache and related functions. The algorithm is the one
used by git, modified so it fits better with the library's functions.
