ui-shared: remove toggle_ssdiff arg to cgit_diff_link()

This argument is never used with a value other than zero, so remove it
and simplify the code.

Signed-off-by: John Keeping <john@keeping.me.uk>
Change "ss" diff flag to an enum

This will allow us to introduce a new "stat only" diff mode without
needing an explosion of mutually incompatible flags.

The old "ss" query parameter is still accepted in order to avoid
breaking saved links, but we no longer generate any URIs using it;
instead the new "dt" (diff type) parameter is used.

Signed-off-by: John Keeping <john@keeping.me.uk>
ui-diff: add "stat only" diff type

This prints the diffstat but stops before printing (or generating) any
of the body of the diff.

No cgitrc option is added here so that we can wait to see how useful
this is before letting people set it as the default.

Suggested-by: Konstantin Ryabitsev <mricon@kernel.org>
Signed-off-by: John Keeping <john@keeping.me.uk>