problem with checking out staging branch

[Janek Warchoł]

Hi,

i have problems with staging branch.  I understand what it's supposed
to do and how branches work; the problem is purely technical.
Firstly, I cannot diff it, unlike master branch: calling 'git diff
origin/master' shows me a diff between the local branch i'm on and the
master branch from remote origin, but calling 'git diff
origin/staging' says

fatal: ambiguous argument 'origin/staging': unknown revision or path
not in the working tree.
Use '--' to separate paths from revisions

-> ?

I concluded that i have to add staging branch ("tell git about its
existence"); i tried

git remote add -ft staging -m staging origin git://git.sv.gnu.org/lilypond.git/

(in analogy to what is written in
http://lilypond.org/doc/v2.15/Documentation/contributor/downloading-remote-branches
about downloading master), but git protested

fatal: remote origin already exists.

i suppose this means that 'git remote' is used for whole "locations
(references)", not branches.  How can i add a branch, then?  I've
searched CG but found nothing.  I finally tried 'git checkout staging'
and surprisingly it worked, but git message was a bit alarming:

Branch staging set up to track remote branch staging from testorigin.
Switched to a new branch 'staging'

what is "testorigin"?  It is possible that it was something i
accidentally set up 3 months ago, i don't remember; nevertheless this
doesn't make much sense to me.
Can you help?

cheers,
Janek