two branches: keep one difference but merge others forth and back

Hi,

Is this possible.

I'd like to have two branches.

If possible I would be able to merge forth and back between both of them.

However I would like, that certain differences will be kept between both branches.

Is there any way to tell git to permanently ignoring certain commits from merging?

Example:
---------
   Normally shell scripts would have a first line of
'#!/bin/bash',
but in a certain branch I would like that the first lines would be

'#!/usr/local/bin/bash'

All from then on however I'd like to be able to commit on both branches and to merge from the other branches (and always keep this difference)

What I tried:
-------------
My first naive approach was:
- create shell scripts in master,
- create then a branch named 'my_shell'
- modify first lines of shell scripts in this branch and commit
- checkout master
- merge my_shell to master with merge strategy 'ours'
  git pull my_shell -s ours
- now I changed something else in master
- when I try to merge back to branch my_shell I will not only get
  the most recent changes done in master, but I will also undo the
changes in line 1 of my shell scripts.

So it seems I am not doing things as one should.

Potential other strategies:
----------------------------
- never commit anything on branch my_shell and just pull regularly
  from master to keep it synced.

- commit changes / bug fixes also on branch my_shell, but NEVER merge
back to master. If a change grom my_shell is really needed on master,
then just cherry-pick.

Thanks in advance for suggestions how you would deal with such 'situations'