branch -r -v: do not spit out garbage

branch -r -v: do not spit out garbage

The codepath to emit relationship between the branch and what it tracks
forgot to initialize a string buffer stat[] to empty when showing a
tracking branch.  This moves the emptying so that the buffer starts as
empty and stays so when no information is added to fix this issue.

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