Declare forward-compatibility with libgit2 v1.3.0 #minor

This commit marks this (Golang) version of git2go as being compatible
with libgit2 v1.3.0.

Allow building libgit2 with Chromium zlib

This change allows the caller to set the `USE_CHROMIUM_ZLIB=ON`
environment variable to use the Chromium implementation of zlib when
building libgit2.

Add support for managed SSH transport

This change drops the dependency on libssh2 and instead uses Go's
implementation of SSH.

Add the version to the go.mod file

This change makes it possible to consume this library with a version.

Uprev vendor/libgit2 to v0.99

This uprevs libgit2 to the latest and greatest.

Add support for Odb.MultiPackIndex()

This change exposes the binding for `git_odb_write_multi_pack_index()`.

Add the /v28 suffix to go.mod

This is compliant with
https://github.com/golang/go/wiki/Modules#releasing-modules-v2-or-higher,
which says

```
Major branch: Update the go.mod file to include a /v3 at the end of the
module path in the module directive (e.g., module
github.com/my/module/v3). Update import statements within the module to
also use /v3 (e.g., import "github.com/my/module/v3/mypkg"). Tag the
release with v3.0.0.
```

This also means that unfortunately we cannot keep using libgit2's exact
version number (plus a build number for git2go purposes, since go only
recognizes vMAJOR.MINOR.PATCH format).

(Tentatively) fixes: libgit2#536

Merge pull request libgit2#531 from suhaibmujahid/patch-1

Switch over the version constraints to v0.28

Merge pull request libgit2#530 from libgit2/cmn/bump-libgit2-28

Update libgit2 to v0.28.4

Merge pull request libgit2#529 from libgit2/cmn/bump-libgit2-27

Update libgit2 to v0.27.10