Register a filter under a given name with a given priority.
As mentioned elsewhere, the initialize callback will not be invoked immediately. It is deferred until the filter is used in some way.
A filter's attribute checks and check and apply callbacks will be
issued in order of priority on smudge (to workdir), and in reverse
order of priority on clean (to odb).
Two filters are preregistered with libgit2:
Currently the filter registry is not thread safe, so any registering or deregistering of filters must be done outside of any possible usage of the filters (i.e. during application setup or shutdown).
A name by which the filter can be referenced. Attempting to register with an in-use name will return GIT_EEXISTS.
The filter definition. This pointer will be stored as is by libgit2 so it must be a durable allocation (either static or on the heap).
The priority for filter application
0 on successful registry, error code < 0 on failure