update-copyright: accept new option: UPDATE_COPYRIGHT_USE_INTERVALS=2
* build-aux/update-copyright: When UPDATE_COPYRIGHT_USE_INTERVALS=2,
convert a sequence with gaps to the minimal containing range.
For example, convert 2000, 2004-2007, 2009 to 2000-2009.
* tests/test-update-copyright.sh: Test for this.
The FSF confirmed it is ok to do this, assuming there is at
least one significant change per year in the affected range:
http://thread.gmane.org/gmane.comp.lib.gnulib.bugs/29554/focus=29860