Bump libgit2 to latest master
Remove +[GTRemote isValidURLString:]

git_remote_valid_url() disappeared in
libgit2/libgit2@ba67c0752269ba06523fe7f5fa350e6328b20241.
Remove “skip certificate check” option

The libgit2 facility for skipping certificate checks was removed and
replaced with a much more general—but harder to use—API, so it’s easier
to remove this for now.

See the libgit2 CHANGELOG for more information.
Fix argument order to git_graph_ahead_behind

The previous order was a bug, and got fixed in libgit2/libgit2#2539.
Merge pull request #410 from libgit2/update-libgit2

Update libgit2