Milder CVS problems, redux

[JD Smith]

I had previously reported problems with CVS commands in Emacs 22
timing out, and/or the cvs process getting killed.  They didn't recur,
but I *do* now commonly get empty results from the first run of cvs
log (C-x v l) on a local repository, which causes a two line empty
buffer to display.  Re-running the command usually succeeds, but it
seems very likely to me that these problems may be related, perhaps to
some underlying race condition.  

I see that CVS log is running asynchronously (vc-cvs.el):

  (vc-cvs-command
   buffer
   (if (and (vc-stay-local-p file) (fboundp 'start-process)) 'async 0)
   file "log"))

Explicitly removing 'async "fixes" this problem, but I presume log is
async for a reason, and that this shouldn't introduce race conditions
of this sort.  I see that diff can operate asynchronously as well, but
that others do not, which is consistent with my earlier findings
("log" and "diff" were the problem cases).  Unfortunately I don't have
much experience debugging asynchronous processes.  I can consistently
reproduce the "blank buffer" issue on C-x v l as of now.