Utils: Split FilePath::fileName

Into the regular case, and a fileNameWithPathComponents() function
handling the rarely used case with argument.

This saves a few cycles in the regular case and makes the function
usable by member function pointer in e.g. Utils::transform.

Change-Id: Ic4469198c3f8a44f59e50d70e67b864d876700d5
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/libs/utils/fileutils.cpp

@@ -736,7 +736,13 @@ QString FilePath::toUserOutput() const
     return m_url.toString();
 }
 
-QString FilePath::fileName(int pathComponents) const
+QString FilePath::fileName() const
+{
+    const QChar slash = QLatin1Char('/');
+    return m_data.mid(m_data.lastIndexOf(slash) + 1);
+}
+
+QString FilePath::fileNameWithPathComponents(int pathComponents) const
 {
     if (pathComponents < 0)
         return m_data;