Warn when signature cannot be properly inferred from configuration

Author: nulltoken

LibGit2Sharp/Configuration.cs

@@ -368,6 +368,8 @@ private string NormalizeUserSetting(bool shouldThrowIfNotFound, string entryName
                 throw new LibGit2SharpException(message);
             }
 
+            Log.Write(LogLevel.Warning, message);
+
             return defaultValue();
         }