Refactor all of the push option support code to allocate the list
immediately before parsing the options section off the stream.
Move option support down to ReceivePack instead of BaseReceivePack.
Push options are specific to the ReceivePack protocol and are not
likely to appear in the 4 year old subscription proposal. These
changes are OK before JGit 4.5 ships as no consumer should be relying
on these new APIs.
Change-Id: Ib07d18c877628aba07da07cd91875f918d509c49