Skip blame libgit2 test if not in libgit2 repo

One blame test replies 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.
Add warning when skipping blame test
Clar: skip tests
Merge pull request #2239 from libgit2/vmg/clar-skip-test

Skip tests on Clar