fix face magit-branch's doc-string

It is not only used for the current branch
as the doc-string used to say.
add magit-get-{current,next}-tag
add magit-insert-status-line

Use it to make magit-refresh-status prettier.  New variable
magit-status-line-align-to controls how to align the "values"
in those status lines.
show current and next tag in status buffer

Show them on a "header line" using new face `magit-tag'.  If there are
no such tags don't insert anything.  The number of commits they are
ahead/behind HEAD are also shown.
Merge branch 'taglines'