Improve "git branch --tracking" output

Improve "git branch --tracking" output

An earlier patch always spelled the full name of the ref that we track
(e.g. "refs/heads/frotz" instead of just "frotz" when we mean the branch
whose name is "frotz").  Worse yet, because we now use the true name of
the ref at the original repository when talk about a tracking branch that
copies from a remote, such a full name alone still does not give enough
information.

This reorganizes the verbose codepath to:

 - differentiate "refs/heads/something" and everything else; we say that
   the branch tracks "branch <something>" if it begins with "refs/heads/",
   and otherwise the branch tracks "ref refs/<someother>/<something>";

 - report the name of the remote when we talk about a tracking branch, by
   saying "branch frotz from origin";

 - not say "by merging" at the end; it is the default and is not worth
   reporting.

Signed-off-by: Junio C Hamano <junio@pobox.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>