merge fdl.texi into magit.texi

Previously fdl.texi was not actually included in magit.texi.  Instead of
including it this patch instead copies the license right into magit.text.
This removes clutter from the top directory, and replacing the text when
moving to a newer version of the license isn't really that much work.
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'
Merge branch 'maint'