UploadPack: Return correct peer user agent on v2 requests

UploadPack: Return correct peer user agent on v2 requests

UploadPack.getPeerUserAgent() doesn't produce the expected results for
protocol v2 requests. In v2, the agent reported in the request (in an
"agent=" line) is not in the clientCapabilities but in a field on its
own. This makes getPeerUserAgent default to the transport user agent.

Making "agent" a shared property between protocol v0/v1 and v2 fixes the
problem, simplifies the function and harmonizes the implementation
between protocol versions.

In a follow up commit the "agent" will be identified on parsing time,
instead of taking it from the client capabilities.

Change-Id: Idf9825ec4e0b81a1458c8e3701f3e28aafd8a32a
Signed-off-by: Ivan Frade <ifrade@google.com>