To help listing the available commands, using the same formatting as
already used in `fluxcd/flux2`.

Signed-off-by: Hidde Beydals <hello@hidde.co>

- Detection of the system version of `libgit2` is attempted using
  `pkg-config`. If this fails, or does not match the version as defined
  in the `Makefile` (or configured using `LIBGIT2_VER=1.x.y`), the
  library is compiled using the instructions from the `BASE_IMG`, and
  installed to `REPOSITORY_ROOT/hack/libgit2`.
- Where `libgit2` is required as a dependency, `LD_LIBRARY_PATH` and/or
  `PKG_CONFIG_PATH` instructions are added as a prefix to the command.
  This works for system libraries as well, because the paths are
  ignored if they do not exist.

Signed-off-by: Hidde Beydals <hello@hidde.co>

This commit updates `github.com/libgit2/git2go` to `v31.6.1` (with
`libgit2` `1.1.1`), and changes the container image build process so
that it makes use of `ghcr.io/hiddeco/golang-with-libgit2`.

This image provides a pre-build dynamic `libgit2` dependency linked
against OpenSSL and LibSSH2 (without gcrypt), and a set of cross-compile
build tools (see
[rationale](https://github.com/hiddeco/golang-with-libgit2#rationale) and
[usage](https://github.com/hiddeco/golang-with-libgit2#usage) for more
detailed information).

The linked set of dependency should solve most known issues around
unsupport private key types, but does not resolve the issues with ECDSA*
and ED25519 hostkeys yet. Solving this requires a newer version of
`libgit2` (`>=1.2.0`), which currently does not seem to work properly
with `git2go/v32`.

Signed-off-by: Hidde Beydals <hello@hidde.co>

This moves the `libgit2` compilation to the image, to ensure it
can be build on builders that aren't backed by AMD64.

The image is structured in such a way that e.g. running nightly
builds targeting a different Go version, or targeting a different
OS vendor would be possible in the future via build arguments.

Signed-off-by: Hidde Beydals <hello@hidde.co>

This ensures the Dockerfile used for testing is making use of the
same scratch image to compile `libgit2` as the actual application
image.

In a future iteration we should restructure our GitHub Action
workflows to re-use the application image, saving us an additional
Dockerfile and a duplicate build. Inspiration for this (which makes
use of a local registry for the duration of the build) can be found
at: https://github.com/fluxcd/golang-with-libgit2/blob/main/.github/workflows/build.yaml

Signed-off-by: Hidde Beydals <hello@hidde.co>

Signed-off-by: Hidde Beydals <hello@hidde.co>

This includes a tiny fix for Darwin to ensure the generated `.pc`
file includes the right paths.

Signed-off-by: Hidde Beydals <hello@hidde.co>