in find_paragraph(), add parameter begin, the line that we can't move

in find_paragraph(), add parameter begin, the line that we can't move
further back than when searching for a paragraph; this is needed to
ensure that we don't justify the same lines more than once if
auto-indent is turned on, and the indentation of what should be the
previous paragraph matches that of what should be the current paragraph

git-svn-id: svn://svn.savannah.gnu.org/nano/trunk/nano@3224 35c25a1d-7b9e-4130-9fde-d3aeb78583b8