libgit2: update to 1.8.1.

This release primarily includes straightforward bugfixes, as well
as new functionality to have more control over the HTTP User-Agent
header. However, there is an API change from v1.8 that was required
for cross-platform compatibility.

In v1.8, libgit2 introduced the report_unchanged member in the
git_fetch_options structure. We mistakenly introduced this as a
bitfield, which is not suitable for our public API. To correct this
mistake, we have removed the report_unchanged member. To support
the report unchanged tips option, users can set the update_fetchhead
member to include the GIT_REMOTE_UPDATE_REPORT_UNCHANGED value.

The libgit2 projects regrets the API change, but this was required
to support cross-platform compatibility.

What's Changed

New features

    Allow more control over the user-agent by @ethomson in #6788

Bug fixes

    commit: Fix git_commit_create_from_stage without author and committer by @florianpircher in #6781
    process.c: fix environ for macOS by @barracuda156 in #6792
    Bounds check for pack index read by @ConradIrwin in #6796
    transport: provide a useful error message during cancellation by @ethomson in #6802
    transport: support sha256 oids by @ethomson in #6803
    Revparse: Correctly accept ref with '@' at the end by @csware in #6809
    remote: drop bitfields in git_remote_fetch_options by @ethomson in #6806
    examples: fix memory leak in for-each-ref.c by @qaqland in #6808
    xdiff: use proper free function by @ethomson in #6810
    rand: avoid uninitialized loadavg warnings by @ethomson in #6812
    cli: include alloca on illumos / solaris / sunos by @ethomson in #6813
    Update git_array allocator to obey strict aliasing rules by @ethomson in #6814
    tree: avoid mixed signedness comparison by @ethomson in #6815

Build and CI improvements

    ci: update nightly workflows by @ethomson in #6773
    ci: give all nightly builds a unique id by @ethomson in #6782
    cmake: remove workaround that isn't compatible with Windows on ARM by @hackhaslam in #6794

Documentation improvements

    Docs meta-updates by @ethomson in #6787

Dependency updates

    Enable llhttp for HTTP parsing by @sgallagher in #6713

(wiz)