Move the BaseReceivePack implementation back into ReceivePack. This is a
backward-incompatible change. For example, BaseReceivePack.FirstLine no
longer exists and cannot be referenced. However, most of the code
should just work by replacing BaseReceivePack with ReceivePack.
Although this is an API change, it only affects callers using JGit as a
server, and there are very few of those in the wild.
Change-Id: I1ce92869435d5eebb7d671be44561e69c6233134
Signed-off-by: Masaya Suzuki <masayasuzuki@google.com>