Skip test_blame_simple__trivial_libgit2 if not in libgit2 repo

The `test_blame_simple__trivial_libgit2` test relies on being run from within the libgit2 repository to leverage having a longer history to play with, but some bundled versions of libgit2 don't have the whole libgit2 history.  This just skips that test if the repository can't be opened successfully.

Fixes #2199