Which file is older in history?

Hi,

I want to compare two files in a git repository and tell which one was
introduced into the repository earlier, assuming that they're in the
same history line (by ancestry, not timestamp).  The naive way to do
this is to find the individual commits that introduced the files
(`rev-list HEAD -- <filename> | tail -n 1`), and query whether there
are commits between them (`rev-list commit1 ^commit2`). However, this
is slow.  What about bisecting the history until we find a tree that
contains one file but not the other?

Thanks.

Ram