branch: split off function that writes tracking info and commit subject

branch: split off function that writes tracking info and commit subject

Introduce a add_verbose_info function that takes care of adding

 - an abbreviated object name;
 - a summary of the form [ahead x, behind y] of the relationship
   to the corresponding upstream branch;
 - a one line commit subject

for the tip commit of a branch, for use in "git branch -v" output.

No functional change intended.  This just unindents the code a little
and makes it easier to skip on first reading.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>