Merge branch 'sb/submodule-parallel-fetch' into maint

Fix recently introduced codepaths that are involved in parallel submodule operations, which gave up on reading too early, and could have wasted CPU while attempting to write under a corner case condition. * sb/submodule-parallel-fetch: hoist out handle_nonblock function for xread and xwrite xwrite: poll on non-blocking FDs xread: retry after poll on EAGAIN/EWOULDBLOCK