magit-insert-wazzup-commits: cleanup
magit-insert-wazzup-commits: insert focused branch

Insert the focused branch in the list of branches and mark it with an
asterisk.
magit-insert-wazzup-commits: use overlays for branch labels
remove magit-insert-wazzup-head-line

The focused, or head, branch is now part of the buffer list and
explicitly marked as such.  So we no longer need an extra header
line.
Merge branch 'n/wazzup-1' into next