libgit2: upgrade 1.8.1 -> 1.8.2

libgit2: upgrade 1.8.1 -> 1.8.2

Changelog (https://github.com/libgit2/libgit2/releases/tag/v1.8.2):

- This release reverts a const-correctness change introduced in
v1.8.0 for the git_commit_create functions. We now retain the
const-behavior for the commits arguments from prior to v1.8.0.

This change was meant to resolve compatibility issues with bindings
and downstream users.

What's Changed

New features
- Introduce a stricter debugging allocator for testing by ethomson in #6811

Bug fixes
- Fix constness issue introduced in #6716 by ethomson in #6829

Build and CI improvements
- README: add experimental builds to ci table by ethomson in #6816

Signed-off-by: Trevor Gamblin <tgamblin@baylibre.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>