Support agent= capability in wire protocol

Support agent= capability in wire protocol

Since git-core ff5effd (v1.7.12.1) the native wire protocol transmits
the server and client implementation and version strings using
capability "agent=git/1.7.12.1" or similar.

Support this in JGit and hang the implementation data off UploadPack
and ReceivePack.  On HTTP transports default to the User-Agent HTTP
header until the client overrides this with the optional capability
string in the first line.

Extract the user agent string into a UserAgent class under transport
where it can be specified to a different value if the application's
build process has broken the Implementation-Version header in the
JGit package.

Change-Id: Icfc6524d84a787386d1786310b421b2f92ae9e65