git with custom diff for commits

Hello folks,

I do a nightly mysqldump of a database and check it into a git
repository. mysqldump generates a timestamp as part of that output
which is causing git to think that the file changes every night when
it really doesn't. The timestamp is simply in an SQL comment.

So what I'd like to do is teach git to ignore that particular SQL
timestamp comment. I've tried to set up an external diff script that
runs diff -I "<<sql timestamp comment>>" that effectively ignores the
timestamp. While this works with "git diff", it seems when git
commits, it still sees the differences.

How do I properly teach git to ignore these types of differences?

Gerald.