patch, 4 partsSpend more effort in git-fetch-pack finding common revisions

This series of 4 patches extends the fetch-pack/upload-pack protocol such that more than just one common revision is used to find out which objects are needed by the client.

This means that the client spends more time on enumerating candidates, and the server just acknowledges those it knows about. That is done until the client runs out of candidates, or the server got HAS_MAX common revisions.

While it does not make much of a difference when you are tracking one linear branch on one repository, this change should make it more efficient (and also ease the burden on the server) to work with several repositories having several branches, and intertwined development.

Next thing I'll do is write some nasty test cases.

Flame on, Dscho