Junio C Hamano [off-list ref] writes:
If the topic is too disruptive, however, they may have to synthesize
a good base (i.e. "merge on top of 'master' topics A, B and C, and
then queue these patches") but it may not be possible (e.g. among
the topics it interacts with, some topic may not yet be stable to
build on), in which case they must catch up quickly every time these
other topics it depends on are updated to avoid being left behind.
If the contributor cannot afford the time for quicker turnaround
that way, it would very much help everybody to announce that they
tentatively withdraw the topic until the other topics get more
stable, after which time they will send an updated version. It
will help everybody, including those who want to build on top of
what the series adds.
Oh, another thing. Such a contributor who is blocked by topics by
others can help their own series by helping these other topics get
polished, i.e. with additional reviews, additional test reports,
etc.