global: adjust init count under lock

Our global initialization functions `git_libgit2_init()` and
`git_libgit2_shutdown()` both adjust a global init counter to determine
whether we are the first respectively last user of libgit2. On
Unix-systems do not do so under lock, though, which opens the
possibility of a race between these two functions:

    Thread 1                            Thread 2
                            git__n_inits = 0;
    git_libgit2_init();
    git_atomic_inc(&git__n_inits);
    /* git__n_inits == 1 */
                                        git_libgit2_shutdown();
                                        if (git_atomic_dec(&git__n_inits) != 0)
                                            /* git__n_inits == 0, no early exit here */
                                        pthread_mutex_lock(&_init_mutex);
                                        shutdown_common();
                                        pthread_mutex_unlock(&_init_mutex);
    pthread_mutex_lock(&_init_mutex);
    init_once();
    pthread_mutex_unlock(&_init_mutex);

So we can end up in a situation where we try to shutdown shared data
structures before they have been initialized.

Fix the race by always locking `_init_mutex` before incrementing or
decrementing `git__n_inits`.

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