Add for_each_remote() function, and extend remote_find_tracking()

Add for_each_remote() function, and extend remote_find_tracking()

The function for_each_remote() does exactly what the name
suggests.

The function remote_find_tracking() was extended to be able to
search remote refs for a given local ref.  The caller sets
either src or dst (but not both) in the refspec parameter, and
remote_find_tracking() will fill in the other and return 0.

Both changes are required for the next step: simplification of
git-branch's --track functionality.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>