Release GIL in Walker.__next__

The first call to git_revwalk_next may take a while to complete in large repositories -- sometimes a couple of seconds.

Release the GIL during git_revwalk_next to avoid locking up multithreaded applications. (Note: cffi wraps C function calls in a similar fashion.)

src/walker.c

@@ -149,7 +149,10 @@ Walker_iternext(Walker *self)
     git_commit *commit;
     git_oid oid;
 
+    Py_BEGIN_ALLOW_THREADS
     err = git_revwalk_next(&oid, self->walk);
+    Py_END_ALLOW_THREADS
+
     if (err < 0)
         return Error_set(err);