Using a `git_oid *` in filter options was a mistake; it is a deviation
from our typical pattern, and callers in some languages that GC may need
very special treatment in order to pass both an options structure and a
pointer outside of it.

Introduce `GIT_ATTR_CHECK_INCLUDE_COMMIT`, which like 4fd5748 allows
attribute information to be read from files in the repository.  4fd5748
always reads the information from HEAD, while
`GIT_ATTR_CHECK_INCLUDE_COMMIT` allows users to provide the commit to
read the attributes from.

Allow more advanced attribute queries using a `git_attr_options`, and
extended functions to use it.  Presently there is no additional
configuration in a `git_attr_options` beyond the flags, but this is for
future growth.

Stop returning a void for functions, future-proofing them to allow them
to fail.

When `GIT_ATTR_CHECK_INCLUDE_HEAD` is specified, read `gitattribute`
files that are checked into the repository at the HEAD revision.

Our enumeration values are not generally suffixed with `T`.  Further,
our enumeration names are generally more descriptive.

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.