Submodules don't exist in the objectdb and the code is making us try to
look for a blob with its commit id, which is obviously not going to
work.

Skip the test if the user wants to insert a submodule.

ssh: initialize libssh2

We should have been doing this, but it initializes itself upon first
use, which works as long as nobody's doing concurrent network
operations. Initialize it on our init to make sure it's not getting
initialized concurrently.

Expand OpenSSL and libssh2 thread safety documentation

test: make sure we retry the auth callback on all platforms