diff: complete support for git patchid

Instead of using a signed type (`off_t`) use a new `git_object_size_t`
for the sizes of objects.

Git is generating patch-id using a stripped down version of a patch
where hunk header and index information are not present.

Signed-off-by: Gregory Herrero <gregory.herrero@oracle.com>

Using an `enum` causes trouble when used with C++ as bitwise operations are not possible w/o casting (e.g., `opts.flags &= ~GIT_BLOB_FILTER_CHECK_FOR_BINARY;` is invalid as there is no `&=` operator for `enum`).

Signed-off-by: Sven Strickroth <email@cs-ware.de>

In libgit2 nomenclature, when we need to verb a direct object, we name
a function `git_directobject_verb`.  Thus, if we need to init an options
structure named `git_foo_options`, then the name of the function that
does that should be `git_foo_options_init`.

The previous names of `git_foo_init_options` is close - it _sounds_ as
if it's initializing the options of a `foo`, but in fact
`git_foo_options` is its own noun that should be respected.

Deprecate the old names; they'll now call directly to the new ones.

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 C89 standard states in §6.7.2.2 "Enumeration specifiers":

> The expression that defines the value of an enumeration constant shall
> be an integer constant expression that has a value representable as an
> int.

On most platforms, this effectively limits the range to a 32 bit signed
integer. The enum `git_diff_option_t` though recently gained an entry
`GIT_DIFF_INDENT_HEURISTIC = (1u << 31)`, which breaks this limit.

Fix the issue by using a gap in `git_diff_option_t`'s enum values. While
this has the benefit of retaining our API, it may break applications
which do not get recompiled after upgrading libgit2. But as we are
bumping the soversion on each release anyway and thus force a recompile
of dependents, this is not a problem.