Getting an actuallt useful merge base?

Hello,

I find the results of git merge-base A B quite useless.

Suppose you have a repository with file sets

S and T

where S are sources which are developed in mainline and number of stable
versions, and feature branches, and T are build tools (such as autoconf
tests or whatever) that are largely independent of the source version.

Because of the independence of T from S T are developed in a separate
branch t which is merged into all branches developing S as needed.

Fixes to S may affect more than one version, and depending on the
situation it might be useful to apply fixes to S to mutiple
stable/feature branche at once. For that one would need a merge base of
the branches in question.

However, merge-base almost always give a commit on branch t which is the
merge base of files in set T and does not contain files in set S at all.
In other words it is merge base only for files from set T and not set S.
Can I get merge base that is merge base for all files that have common
history between two branches?

Thanks

Michal