"disabling bitmap writing, as some objects are not being packed"?

[David Turner <novalis@novalis.org>]

I'm a bit confused by the message "disabling bitmap writing, as some
objects are not being packed".  I see it the my gc.log file on my git
server.

1. Its presence in the gc.log file prevents future automatic garbage
collection.  This seems bad.  I understand the desire to avoid making
things worse if a past gc has run into issues.  But this warning is
non-fatal; the only consequence is that many operations get slower.  But
a lack of gc when there are too many packs causes that consequence too
(often a much worse slowdown than would be caused by the missing
bitmap).

So I wonder if it would be better for auto gc to grep gc.log for fatal
errors (as opposed to warnings) and only skip running if any are found.
Alternately, we could simply put warnings into gc.log.warning and
reserve gc.log for fatal errors. I'm not sure which would be simpler.  

2. I don't understand what would cause that message.  That is, what bad
thing am I doing that I should stop doing?  I've briefly skimmed the
code and commit message, but the answer isn't leaping out at me.