Let the caller supply the diff text to diffstat()

Let the caller supply the diff text to diffstat()

Almost all diffstat() callers already have the diff text, so they
might as well pass it to diffstat() instead of letting it recompute
it. In some cases this even makes for a code simplification since the
diff (and thus diffstat) parameters were nontrivial.

Also, diffstat() wasn't as versatile as diff(); for example, it didn't
accept any extra diff options. This patch solves all of those problems
as a side-effect.

Signed-off-by: Karl Hasselström <kha@treskal.com>