solaris: diff -u emits "No differences encountered"

Solaris diff -u isn't silent when two files are identical, and tests that
don't account for that will fail.  Fix those tests, and introduce a check
that prevents reintroduction.

contrib/check-code.py

@@ -121,6 +121,7 @@ testpats = [
     (r'^( *)\t', "don't use tabs to indent"),
     (r'sed (-e )?\'(\d+|/[^/]*/)i(?!\\\n)',
      "put a backslash-escaped newline after sed 'i' command"),
+    (r'^diff *-\w*u.*$\n(^  \$ |^$)', "prefix diff -u with cmp"),
   ],
   # warnings
   [

tests/test-convert-bzr.t

@@ -102,7 +102,7 @@ compare timestamps
   >   > ../bzr-timestamps
   $ cd ..
   $ hg -R source-hg log --template "{date|isodate}\n" > hg-timestamps
-  $ diff -u bzr-timestamps hg-timestamps
+  $ cmp bzr-timestamps hg-timestamps || diff -u bzr-timestamps hg-timestamps
   $ cd ..
 
 merge

tests/test-highlight.t

@@ -522,7 +522,7 @@ hgweb fileannotate, raw
   $ echo "" >> b
   $ echo "" >> b
   $ echo "" >> b
-  $ diff -u b a
+  $ cmp b a || diff -u b a
 
 hgweb filerevision, raw
 
@@ -531,7 +531,7 @@ hgweb filerevision, raw
   $ echo "200 Script output follows" > b
   $ echo "" >> b
   $ hg cat primes.py >> b
-  $ diff -u b a
+  $ cmp b a || diff -u b a
 
 hgweb highlightcss friendly