Add/multiply with overflow tweaks

Improve deprecation of old enums

Correct the documentation on the fallback add/multiply with overflow
functions.

Windows provides <intsafe.h> which provides "performant" add and
multiply with overflow operations.  Use them when possible.

Use the smallest unsigned type that is equivalent to `size_t` to
simplify the conditionals.  Error if we're on a system that we believe
offers builtins but we cannot determine which one to use.

The C standard does not specify whether an enum is a signed or unsigned
type.  Obviously, any enum that includes negative values _must_ be
signed, but if all values are positive then the compiler is free to
choose signed or unsigned.

Thus, by changing the type signatures to `git_object_t` and declaring
the old `GIT_OBJ_` values as a signed or unsigned int, we risk a
mismatch between what the compiler has chosen for a `git_object_t`'s
type and our type declaration.

Thus, we declare the deprecated values as the enum instead of guessing.

Recent GCC enables `-Wunused-const-variables`, which makes output quite
noisy.  Disable unused warnings for our deprecated variables.

Move `git_ref_t` to `git_reference_t`

More `git_obj` to `git_object` updates

ci: only run invasive tests in nightly

Always build a cdecl library

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.

To explicitly break end-users who were specifying STDCALL, explicitly
fail the cmake process to ensure that they know that they need to change
their bindings.  Otherwise, we would quietly ignore their option and the
resulting cdecl library would produced undefined behavior.

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.

changelog: document changes since 0.27

Fix a bunch of warnings

mailmap: prefer ethomson@edwardthomson.com

My @github.com address is no more.

Update some missed types that were continuing to use the old `GIT_OBJ`
names.

We use the term "invalid" to refer to bad or malformed data, eg
`GIT_REF_INVALID` and `GIT_EINVALIDSPEC`.  Since we're changing the
names of the `git_object_t`s in this release, update it to be
`GIT_OBJECT_INVALID` instead of `BAD`.

Mark old object type values as `static const` so that we can mark them
as deprecated with the `GIT_DEPRECATED` macro.

Update internal usage to use the `git_reference` names for constants.

Update the reference type names from an abbreviation (`git_ref`) to use
the fullname (`git_reference`).  This ensures that we are consistent
with our naming for reference types and functions throughout the
library.

The previous names are now marked as deprecated.

Convert tests/resources/push.sh to LF endings

This changes that file to use UNIX line-endings, which makes sense since
this is a UNIXy file.

Get rid of some test files that were accidentally committed