Update to libgit2 v1.1 and use main branch for tests.

[de-vri-es]

This PR updates the libgit2 submodule to the v1.1.0 tag. It also updates the unit tests to use a fixed name for the HEAD of created test repositories.

Since libgit2 1.1.0, the gitconfig of the environment can change the name of the default branch. If the test do not specify a fixed name for the initial HEAD, they will fail on some systems with different configuration.

To make sure I didn't miss anything, I ran the tests this with init.defaultBranch in my gitconfig set to a value different from both master and main.

As a bonus, this PR also replaces master with main in documentation and examples of the git2 crate.

This should automatically address rust-lang/cargo/issues/8588 once cargo updates the dependency on this crate.

[ehuss]

Thanks! I think the version will also need to be bumped (similar to 218a2b3).

[alexcrichton]

I'm gonna go ahead and merge this and I'll do the version bumps as a follow-up commit for a release.