Reduces the number of callbacks for signing a commit during a rebase operation to just one callback. That callback has 2 out git_buf parameters for signature and signature field. We use git_buf here, because we cannot make any assumptions about the heap allocator a user of the library might be using.

2 callbacks have been added to git_rebase_options, git_rebase_commit_signature_cb and git_rebase_commit_signature_field_cb. When git_rebase_commit_signature_cb is present in git_rebase_options, it will be called whenever git_rebase_commit is performed, giving an opportunity to sign the commit. The signing procedure can be skipped if the callback specifies passthrough as the error. The git_rebase_commit_signature_field_cb will only be called if the other callback is present or did not passthrough, and it provides means to specify which field a signature is for.
Git_rebase_options was chosen as the home for these callbacks as it keeps backwards compatibility with the current rebase api.

Move `giterr` to `git_error`

Update GITERR and giterr usages in the static code analysis tools to use
the new names.

Move to the `git_error` name in the internal API for error-related
functions.

Replace the `GITERR` values with a `const int` to deprecate error
values.

Move to the `git_error` name in error-related functions, deprecating the
`giterr` functions.  This means, for example, that `giterr_last` is now
`git_error_last`.  The old names are retained for compatibility.

This only updates the public API; internal API and function usage
remains unchanged.

odb: Fix odb foreach to also close on positive error code

In include/git2/odb.h it states that callback can also return
positive value which should break looping.

Implementations of git_odb_foreach() and pack_backend__foreach()
did not respect that.

repository: free memory in symlink detection function

ci: update poxyproxy, run in quiet mode

Update the proxy so that we can enable a quiet mode.

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