VCS: Do not try to add files that are already tracked

Task-number: QTCREATORBUG-2455
Change-Id: I4b1fa9676ad4a8d7293e035e4d6103251bef8aee
Reviewed-by: Nikita Baryshnikov <nib952051@gmail.com>
Reviewed-by: Cristian Tibirna <tibirna@kde.org>
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>

src/plugins/git/gitclient.cpp

@@ -972,6 +972,15 @@ QString GitClient::findGitDirForRepository(const QString &repositoryDir) const
     return res;
 }
 
+bool GitClient::managesFile(const QString &workingDirectory, const QString &fileName) const
+{
+    QByteArray output;
+    QStringList arguments;
+    arguments << QLatin1String("ls-files") << QLatin1String("--error-unmatch") << fileName;
+    return fullySynchronousGit(workingDirectory, arguments, &output, 0,
+                               VcsBasePlugin::SuppressCommandLogging);
+}
+
 VcsBase::VcsBaseEditorWidget *GitClient::findExistingVCSEditor(const char *registerDynamicProperty,
                                                                const QString &dynamicPropertyValue) const
 {