git_command: refactor User-Agent settings

Convert the RepoUserAgent function into a UserAgent class.  This
makes it cleaner to hold internal state, and will make it easier
to add a separate git User-Agent, although we don't do it here.

We make the RepoSourceVersion independent of GitCommand so that
it can be called by the class (later).

Bug: https://crbug.com/gerrit/11144
Change-Id: Iab4e1f974b8733a36b243b2d03f5085a96effa19
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/239232
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Mike Frysinger <vapier@google.com>