diffstat: use ui.plain() instead of ui.interactive()

Previously, a default width of 80 is used for non-interactive
sessions. This behaviour was introduced before HGPLAIN was supported.

mercurial/commands.py

@@ -1158,7 +1158,9 b' def diff(ui, repo, *pats, **opts):'
     m = cmdutil.match(repo, pats, opts)
     it = patch.diff(repo, node1, node2, match=m, opts=diffopts)
     if stat:
-        width = ui.interactive() and util.termwidth() or 80
+        width = 80
+        if not ui.plain():
+            width = util.termwidth()
         ui.write(patch.diffstat(util.iterlines(it), width=width,
                                 git=diffopts.git))
     else: