--- a/check_whitespace.py	Fri Mar 06 14:41:19 2015 +0100
+++ b/check_whitespace.py	Fri Mar 20 14:43:48 2015 +0100
@@ -99,21 +99,36 @@
             if line and line[0] in ' +':
                 linenum += 1
 
-def hook(ui, repo, hooktype, node, **kwargs):
+def hook(ui, repo, hooktype, node=None, source=None, **kwargs):
     import os, sys, subprocess
-
-    added = 0
-    for filename, linenum, msg in trailing_whitespace(repo[node].diff()):
-        print >> sys.stderr, ('%s, line %d: %s added' %
-                              (filename, linenum, msg))
-        added += 1
+    if hooktype not in ['pretxnchangegroup', 'pretxncommit']:
+        ui.write('Hook should be pretxncommit/pretxnchangegroup not "%s".' % hooktype)
+        return 1
+    added = False
+    branches = {}
+    for rev in xrange(repo[node], len(repo)):
+        branch = repo[rev].branch()
+        if not branches.has_key(branch):
+            # first time we see this branch, remember parents to diff against
+            branches[branch] = repo[rev].parents()
+        desc = 0
+        for d in repo[rev].descendants():
+            break
+        else:
+            # no descendants for this revision, check diff with saved parents
+            for p in branches[branch]:
+                for filename, linenum, msg in trailing_whitespace(repo[rev].diff(p)):
+                    print >> sys.stderr, ('%s, line %d: %s added' %
+                                          (filename, linenum, msg))
+                    added = True
     if added:
         # save the commit message so we don't need to retype it
-        cmtsv = os.path.join('.hg','commit.save')
-        subprocess.call(['hg', 'tip', '--template', '{desc}'],
-                        stdout=open(cmtsv, 'w'))
-        print >> sys.stderr, 'commit message saved to %s' % cmtsv
-        return True
+        if source != 'serve':
+            cmtsv = os.path.join('.hg','commit.save')
+            subprocess.call(['hg', 'tip', '--template', '{desc}'],
+                            stdout=open(cmtsv, 'w'))
+            print >> sys.stderr, 'commit message saved to %s' % cmtsv
+        return 1
 
 if __name__ == '__main__':
     if hook(None, None, None, None):