Update to libgit2 0.28.2

[cuviper]

ref: Upstream changelog

This includes a semver bump for all crates, as a few APIs have been
renamed in 0.28. Upstream provides deprecated.h aliases, if we'd like
to follow suit, but I feel it's better to just move forward.

• giterr_* -> git_error_*
• GITERR_* -> GIT_ERROR_*
• git_otype -> git_object_t
• GIT_OBJ_* -> GIT_OBJECT_*
• git_ref_t -> git_reference_t
• GIT_REF_* -> GIT_REFERENCE_*
• git_idxentry_* -> git_index_entry_*
• GIT_IDXENTRY_* -> GIT_INDEX_ENTRY_*
• git_buf_free -> git_buf_dispose

Proxy support is now builtin to libgit2, so libcurl is no longer a
dependency at all. Proxies were also the reason for git2-curl to
exist, which seems to be unnecessary for a while now, since libgit2
used libcurl itself since 0.23 anyway. Maybe git2-curl can just be
retired now, but I've updated it just for the sake of compatibility.

The minimum pkg-config libgit2 is now documented and tested for 0.28.
While it previously claimed support for 0.25.1, this was broken by
git_config_entry's include_depth field, which was added after the
0.27 release branched.

[ehuss]

This looks great, thanks for doing this! I'll try to look at it closer sometime soon.

@alexcrichton Do you have any idea why the azure pipeline didn't launch? It was working before.
EDIT: Posted a new PR, and it seemed to work on it, strange!

[ehuss]

I think git2-curl will still be needed by things like Cargo. Cargo performs a variety of configurations on the curl handle (certificate settings, user-agent, debugging, etc.), and those don't seem to have direct analogues in libgit2.

I've been testing this out a bit and I haven't found any problems.

[alexcrichton]

Thanks for the @cuviper! Agreed with @ehuss that we do use the curl support for non-proxy stuff, but it's indeed minor (ish) compared to the proxy support which was critical at the time! In any case looks like this is all ship shape, so I'm gonna merge.

[alexcrichton]

And now published on crates.io!