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.

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

Fix crash on remote connection when GIT_PROXY_AUTO is set but no proxy is detected

Since Coverity is down for a unspecified timeframe, isolate it from the
"hosted" nightlies.

Make ENABLE_WERROR actually work

Remove unconditional -Wno-deprecated-declaration on macOS

After taking into consideration the following, I think this should be
removed :

- OpenSSL isn't the default on Apple platforms
- you have to jump through hoops to get CMake to use OpenSSL on macOS
(headers aren't in `/usr/include`, so you have to provide `-DOPENSSL_*`
overrides)
- users are likely (as getting anywhere near the installed 0.9.8 version
is insanity IMHO) to package a "modern" version, which wouldn't be
marked as deprecated

This change explicitly adds -Werror to the CFLAGS.

Due to the way that the ADD_C_FLAG_IF_SUPPORTED() macro was mangling the
flag name to convert it into a define name, any warning that had a dash
in its name was not being correctly enabled. Additionally, any flag that
is enabled implicitly by the compiler (like -Wunused-result and
-Wdeprecated-declarations) would not cause an error unless they were
explicitly enabled with the ENABLE_WARNINGS() macro.

Fix warning 'function': incompatible types - from 'git_cvar_value *' to 'int *' (C4133) on VS

Fix Linux warnings