haiderali,

This is not a bug or feature request for MantisBT (you are asking for help on how to merge your custom changes). I am therefore resolving this issue as "no change required".

Please use the forums, the mantisbt-help mailing list or IRC to get support on customizing and using MantisBT (refer to http://www.mantisbt.org/support.php for links and further details).

I recommend to use Git for your purposes.
In a first step you can use it to get the diffs between 1.2.15 and 1.2.17
In a second step you can use it it to create your own branch for your custom changes. After this step it will be quite easy when there will be 1.2.18, ...