Managing several threads of the same project in GIT

[pggrd <git@paggard.com>]

Hello,

I've been studying GIT for some time already, but still have not figured out if
it can be used for the scenario like this:

for example:

I have a project, which divides to threads aiming to different clients
most of the code in the project is shared between the threads
in certain files there are different code parts to meet client requirements 
but even those files share the rest of the code

What I need to be able to do is:
- to get code for any thread I like at any time
- to be able to develop each thread separately
- to be able to merge certain changes from one thread to another, at the same
time without loosing the changes specific to the thread, and keep track of merge
history

With GIT strategy to treat content as a single unity - if you will try to merge
branches - everything will be merged thus overwriting the changes that need to
stay. But in the example above, if I make a change in the shared code in one
thread, I need to be able to populate this change to all the threads, but this
change only.

so - is this possible in GIT and if yes - what kind of strategy should be used?

thanks