git-isvn: First cut at a feedback mechanism from brancher -> fetcher.

Goal: We don't want to exhaust memory if the fetchers get too far ahead
of the brancher threads.

So, if the branchers are being "slow," currently defined by a commit
taking a "long time" (500 ms), then fetchers block until a committer is
ready.

This could be a performance issue if the network link is quite slow and
fetchers can't re-fill the buffer in 500 ms AND you have quite frequent
slow commits (slow disk?). However:

Initial testing indicates this isn't quite enough; in particular there
is nothing preventing the fetcher threads from outpacing the commit
thread when commits don't take 500 ms. So they still chew through SVN
history and local memory if the disk isn't keeping up.