safe interface for tracing

[extrawurst]

@alexcrichton like mentioned in #696 I would like to add a safe wrapper for git_trace_set. since it does not take a custom payload I can only imagine doing this via a global variable to store the user callback. this requires Mutex protection which in turn requires lazy_static

a) is that ok for you to add as a dependency?
b) do we have an alternative way to do this?
c) do we say such stuff belongs into user code or a utility crate?

[alexcrichton]

I think this could probably be done with an AtomicUsize rather than a mutex? (since we'd just cast a function pointer to a usize and put the value in the global AtomicUsize)

[extrawurst]

I think this could probably be done with an AtomicUsize rather than a mutex? (since we'd just cast a function pointer to a usize and put the value in the global AtomicUsize)

damn you are right - casting the usize to a pointer - that feels so dirty! I love it - PR incoming