Fix g:qf_shorten_path behaviour

[newtonne]

The docs state that the default for g:qf_shorten_path is 1:

vim-qf/doc/qf.txt

Lines 396 to 398 in 7e65325

	*'g:qf_shorten_path'*
	Value: numeric ~
	Default: 1 ~

But this isn't the case as it was defaulted to 0 in qf.vim.

Additionally, I believe the intention of this call:

call setqflist([], 'r', qf_list_title)

is to clear the qflist whilst retaining the title. However, it doesn't actually clear the qflist because the first argument is ignored:

setqflist({list} [, {action} [, {what}]])               setqflist()
                Create or replace or add to the quickfix list.

                If the optional {what} dictionary argument is supplied, then
                only the items listed in {what} are set. The first {list}
                argument is ignored.  See below for the supported items in
                {what}.

This meant that when g:qf_shorten_path was set to > 0, the list with shortened paths was being appended to the list with full paths, resulting in duplicates.