describe example: fix memory allocation size
We need to allocate memory for sizeof(char *) * ncommits, not just for
ncommits.
Issue detected by GCC's AddressSanitizer.
Rename git_threads_ to git_libgit2_
This describes their purpose better, as we now initialize ssl and some
other global stuff in there. Calling the init function is not something
which has been optional for a while now.