If provided with a null signature, skip adding the signature header and create the commit anyway.

We should clear the error before calling the signing_cb to allow the signing_cb to set its own errors. If the CB did not provide an error, we should set our own generic error before exiting rebase_commit__create

In the case that we want to build merge + commit, cherrypick + commit, or even just build a commit with signing callback, `git_rebase_commit_signature_cb` particular callback should be made more generic. We also renamed `signature_cb` to `signing_cb` to improve clarity on the purpose of the callback (build a difference between a git_signature and the act of signing).

So we've ended up with `git_commit_signing_cb`.

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