Merge branch 'sb/submodule-parallel-fetch'

Add a framework to spawn a group of processes in parallel, and use
it to run "git fetch --recurse-submodules" in parallel.

Rerolled and this seems to be a lot cleaner.  The merge of the
earlier one to 'next' has been reverted.

* sb/submodule-parallel-fetch:
  submodules: allow parallel fetching, add tests and documentation
  fetch_populated_submodules: use new parallel job processing
  run-command: add an asynchronous parallel child processor
  sigchain: add command to pop all common signals
  strbuf: add strbuf_read_once to read without blocking
  xread: poll on non blocking fds
  submodule.c: write "Fetching submodule <foo>" to stderr

wrapper.c

@@ -236,8 +236,24 @@ ssize_t xread(int fd, void *buf, size_t len)
 	    len = MAX_IO_SIZE;
 	while (1) {
 		nr = read(fd, buf, len);
-		if ((nr < 0) && (errno == EAGAIN || errno == EINTR))
-			continue;
+		if (nr < 0) {
+			if (errno == EINTR)
+				continue;
+			if (errno == EAGAIN || errno == EWOULDBLOCK) {
+				struct pollfd pfd;
+				pfd.events = POLLIN;
+				pfd.fd = fd;
+				/*
+				 * it is OK if this poll() failed; we
+				 * want to leave this infinite loop
+				 * only when read() returns with
+				 * success, or an expected failure,
+				 * which would be checked by the next
+				 * call to read(2).
+				 */
+				poll(&pfd, 1, -1);
+			}
+		}
 		return nr;
 	}
 }