quoted

+status.parallelSubmodules::
+     When linkgit:git-status[1] is run in a superproject with
+     submodules, a status subprocess is spawned for every submodule.
+     This option specifies the number of submodule status subprocesses
+     to run in parallel. If unset, it defaults to 1.

Why do we default to 1, instead of e.g. grep.threads defaulting to the
"cores available"?

In my cover letter, I noted that with too many processes, status starts to
slow down (but is still better than the baseline). This is because the
expensive part of status is IO, specifically reading objects from the index.
Much of the speedup of this patch comes from taking advantage of the
ability to do parallel reads on an SSD, rather than splitting up the work
of status. However, this doesn't work with an HDD, so status may
actually be slower than baseline with multiple processes since there is
now scheduling/switching overhead.

quoted

+struct submodule_status_util {
+     int changed, ignore_untracked;

nit: we tend not to group by type, which also makes adding API docs
later easier.

Except that we tend to do that if all the things are really boolean flags, which these are, so maybe:

        unsigned int changed:1,
                     ignore_untracked:1;

?

ack.