Do not show empty differences when the user asks for a VCS comparison.

Instead show an ugly message box.

vcs/vcsdiff.cpp

@@ -63,6 +63,12 @@ VcsDiff::VcsDiff( const VcsDiff& rhs )
     d->baseDiff =  rhs.d->baseDiff;
 }
 
+bool VcsDiff::isEmpty() const
+{
+    return d->diff.isEmpty() && d->leftBinaries.isEmpty() && d->rightBinaries.isEmpty()
+                             && d->leftTexts.isEmpty() && d->rightTexts.isEmpty();
+}
+
 VcsDiff::Type VcsDiff::type() const
 {
     return d->type;

vcs/vcsdiff.h

@@ -121,6 +121,9 @@ public:
     void setType( Type );
     void setContentType( Content );
     VcsDiff& operator=( const VcsDiff& rhs);
+    
+    /** @returns whether there are not changes on the diff */
+    bool isEmpty() const;
 private:
     class VcsDiffPrivate* const d;
 };

vcs/vcspluginhelper.cpp

@@ -255,6 +255,15 @@ void VcsPluginHelper::diffToHead()
     d->plugin->core()->runController()->registerJob(job);
 }
 
+QStringList locationListToString(const QList<VcsLocation>& locations)
+{
+    QStringList ret;
+    foreach(const VcsLocation& loc, locations) {
+        ret.append(loc.localUrl().prettyUrl());
+    }
+    return ret;
+}
+
 void VcsPluginHelper::diffJobFinished(KJob* job)
 {
     KDevelop::VcsJob* vcsjob = dynamic_cast<KDevelop::VcsJob*>(job);
@@ -263,7 +272,14 @@ void VcsPluginHelper::diffJobFinished(KJob* job)
     if (vcsjob) {
         if (vcsjob->status() == KDevelop::VcsJob::JobSucceeded) {
             KDevelop::VcsDiff d = vcsjob->fetchResults().value<KDevelop::VcsDiff>();
-            showVcsDiff(new VCSDiffPatchSource(d));
+            if(d.isEmpty())
+                KMessageBox::error(ICore::self()->uiController()->activeMainWindow(),
+                                   i18n("Cannot show the difference because there were none."),
+                                   i18n("VCS support"));
+            else {
+                VCSDiffPatchSource* patch=new VCSDiffPatchSource(d);
+                showVcsDiff(patch);
+            }
         } else {
             KMessageBox::error(ICore::self()->uiController()->activeMainWindow(), vcsjob->errorString(), i18n("Unable to get difference."));
         }