ReceivePack: Accept shallow lines from Git >= 1.9

ReceivePack: Accept shallow lines from Git >= 1.9

In Git 1.9 (5dbd767601 "support pushing from a shallow clone")
the git-core project intentionally broke the existing send-pack
protocol from shallow clients.

Shallow clients now transmit their shallow information during push,
ahead of the old-new command sequence. JGit must accept these lines
when presented.

To protect the server against clients sending partial history,
require the connectivity check when pushed to by a shallow client.

Change-Id: I46639366b0900052c376091e1688f07def44ab79