trace: suffix the callbacks with `_cb`
The trace logging callbacks should match the other callback naming
conventions, using the `_cb` suffix instead of a `_callback` suffix.
Introduce GIT_CALLBACK macro to enforce cdecl
Since we now always build the library with cdecl calling conventions,
our callbacks should be decorated as such so that users will not be able
to provide callbacks defined with other calling conventions.
The `GIT_CALLBACK` macro will inject the `__cdecl` attribute as
appropriate.
Add payloads, bitmaps to trace API
This is a proposed adjustment to the trace APIs. This makes the
trace levels into a bitmask so that they can be selectively enabled
and adds a callback-level payload, plus a message-level payload.
This makes it easier for me to a GIT_TRACE_PERF callbacks that
are simply bypassed if the PERF level is not set.
