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
The code was already there, so factor it out and let users push an OID
by giving it a reference name. Only refs to commits are
supported. Annotated tags will throw an error.
It's not unusual to want the walker to act on HEAD, so add a
convencience function for the case that the user doesn't already have
a resolved HEAD reference.
