mboxgz: do asynchronous git blob retrievals

This lets the -httpd worker process make better use of time
instead of waiting for git-cat-file to respond.  With 4 jobs in
the new test case against a clone of
<https://public-inbox.org/meta/>, a speedup of 10-12% is shown.
Even a single job shows a 2-5% improvement on an SSD.

1 files changed, 7 insertions, 0 deletions

diff --git a/lib/PublicInbox/HTTP.pm b/lib/PublicInbox/HTTP.pm
index 82817465..5844ef44 100644
--- a/lib/PublicInbox/HTTP.pm
+++ b/lib/PublicInbox/HTTP.pm
@@ -488,6 +488,13 @@ sub busy () {
         ($self->{rbuf} || exists($self->{env}) || $self->{wbuf});
 }
 
+# runs $cb on the next iteration of the event loop at earliest
+sub next_step {
+        my ($self, $cb) = @_;
+        return unless exists $self->{sock};
+        $self->requeue if 1 == push(@{$self->{wbuf}}, $cb);
+}
+
 # Chunked and Identity packages are used for writing responses.
 # They may be exposed to the PSGI application when the PSGI app
 # returns a CODE ref for "push"-based responses