Reset all static variables to NULL in clar's __cleanup
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
notes: honor core.notesRef
Setting core.notesRef allows to change the default notes reference used
by Git. Check if set before using GIT_NOTES_DEFAULT_REF. Fixes #649.