How to perform a safe Merge?

[Sascha Müller]

Hi,

how can I perform a 3-way Merge without (automatically) changing the 
content of any file? I am looking for a way to switch off the 
auto-merging, but still get files updated, which I have not changed.

Like this:
    BASE == MINE => Use THEIRS
    BASE != MINE => Conflict

Thanks in advance,
Sascha