check-code: prepend warning prefix only once, but for each warning

The code adding the prefix is now run once per pattern. It was run once per
file (after the change 3e1e4a8aec1e).
Demonstrate that it is working now by extending the test. Raise two different
warnings, one of them twice.

contrib/check-code.py

@@ -467,6 +467,8 b' def checkfile(f, logfunc=_defaultlogger.'
             else:
                 p, msg = pat
                 ignore = None
+            if i >= nerrs:
+                msg = "warning: " + msg
 
             pos = 0
             n = 0
@@ -500,8 +502,7 b' def checkfile(f, logfunc=_defaultlogger.'
                         bl, bu, br = blamecache[n]
                         if bl == l:
                             bd = '%s@%s' % (bu, br)
-                if i >= nerrs:
-                    msg = "warning: " + msg
+
                 errors.append((f, lineno and n + 1, l, msg, bd))
                 result = False
 

tests/test-check-code.t

@@ -185,12 +185,20 b''
   $ cat > warning.t <<EOF
   >   $ function warnonly {
   >   > }
+  >   $ diff -N aaa
+  >   $ function onwarn {}
   > EOF
   $ "$check_code" warning.t
   $ "$check_code" --warn warning.t
   warning.t:1:
    >   $ function warnonly {
    warning: don't use 'function', use old style
+  warning.t:3:
+   >   $ diff -N aaa
+   warning: don't use 'diff -N'
+  warning.t:4:
+   >   $ function onwarn {}
+   warning: don't use 'function', use old style
   [1]
   $ cat > raise-format.py <<EOF
   > raise SomeException, message