Ignore backup & log files from converting projects to a new Visual Studio version.

Global/VisualStudio.gitignore

@@ -64,3 +64,9 @@ stylecop.*
 ~$*
 *.dbmdl
 Generated_Code #added for RIA/Silverlight projects
+
+# Backup & report files from converting an old project file to a newer
+# Visual Studio version. Backup files are not needed, because we have git ;-)
+_UpgradeReport_Files/
+Backup/
+UpgradeLog.XML