diff.c: use diff_free_queue()

diff.c: use diff_free_queue()

Use diff_free_queue() instead of open-coding it.  This shortens the
code and make it less repetitive.

Note that the second hunk in diff_flush() is interesting, because the
'free_queue' label separates the loop freeing the queue's filepairs
from free()-ing the queue's internal array.  This is somewhat
suspicious, but it was not an issue before: there is only one place
from where we jump to this label with a goto, and that is protected by
an 'if (!q->nr && ...)' condition, i.e. we only skipped the loop
freeing the filepairs when there were no filepairs in the queue to
begin with.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>