dgit: Pass --no-renames to git diff-tree -z, avoiding potential trouble

Without this option, git-diff-tree might detect a rename (or possibly
even a copy).  If it does it prints a different output format with a
status of C or R *and a separate filename*.  The latter is an
additional nul-terminated record and would get dgit's interpreter of
the git diff-tree output out of step.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>