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.

The recommendation from engineers within Microsoft is that libraries
should have a calling convention specified in the public API, and that
calling convention should be cdecl unless there are strong reasons to
use a different calling convention.

We previously offered end-users the choice between cdecl and stdcall
calling conventions.  We did this for presumed wider compatibility: most
Windows applications will use cdecl, but C# and PInvoke default to
stdcall for WINAPI compatibility.  (On Windows, the standard library
functions are are stdcall so PInvoke also defaults to stdcall.)

However, C# and PInvoke can easily call cdecl APIs by specifying an
annotation.

Thus, we will explicitly declare ourselves cdecl and remove the option
to build as stdcall.

The GIT_EXTERN macro needs to provide order-specific attributes; update
users of the GIT_DEPRECATED macro to allow for that.

We have no need to take a non-const reference. This does involve some other work
to make sure we don't mix const and non-const variables, but by splitting what
we want each variable to do we can also simplify the logic for when we do want
to free a new reference we might have allocated.

git_object_t is the future; update the public API to use it.  This will
also ensure that we can build our tests which make use of the old API
without modification (and without compiler warnings).

The two "reserved" bits in `git_object_t` are unused.  They were
included for completeness, but downstream users should never use them
and they should not have been made public.

These values are never set.

With the refactoring of `git_otype` into `git_object_t`, we can remove
these from the new API.  They will remain in the old (deprecated) API
in the unlikely event that someone was using them.

Update the `git_otype` names to reflect our current naming conventions.
`git_otype` is now `git_object_t` and the `GIT_OBJ_*` values are now
`GIT_OBJECT_*` values.

The old macro, enumeration and structure names are retained and simply
set to the new names.

We have various macro, enumeration and structure names that were
introduced (very) early in the project and do not match our current
naming conventions.  For instance: `GIT_IDXENTRY...` flags that
correspond to a structure named `git_index_entry`.

Update these to match the current guidance.  The old macros and
enumeration names are reflected as new macros in order to support
backward compatibility (and do so without warnings for consumers).

Allow merge analysis against any reference

revwalk: Allow changing hide_cb