* LibGit2: amend GitError enum

Since upstream commit
https://github.com/libgit2/libgit2/commit/e9cef7c4b16b2cb572ac19fcd39217f7934cfa99
("http: introduce GIT_ERROR_HTTP") an invalid content type yields a
GIT_ERROR_HTTP instead of a GIT_ERROR_NET error. Update the enum to
include this new error so that the unit test for LibGit2 doesn't fail
with "invalid value for Enum Class: 34".

* LibGit2: add resolve_url to RemoteCallbacksStruct for LibGit2 0.99.0

Upstream commit
https://github.com/libgit2/libgit2/commit/59647e1ad095f80112918971b7bbe05adfaf8c3c
("remote: add callback to resolve URLs before connecting") introduced a
new callback "resolve_url" in LibGit2 0.99.0. Even though it is not
currently used in Julia, it needs to be accounted for to get the correct
size for FetchOptionsStruct. An incorrectly aligned FetchOptionsStruct
leads to error messages like "invalid version 0 on git_proxy_options"
when trying to use the latest LibGit2 version.

* LibGit2: update error message checking for LibGit2 0.99.0

Upstream commit
https://github.com/libgit2/libgit2/commit/b9c5b15a7958ab4ecb83504a6d858efd18610297
("http: use the new httpclient") changed the error message to include
additional quotes. Relax the unit test to allow these if present.

Co-authored-by: Milan Bouchet-Valat <nalimilan@club.fr>
(cherry picked from commit 59a315cac72f048847b85e285ae19ef2dc9514c3)