The source code has to be provided. From this, one can easily generate a diff. The only thing that such a clause will do is having to inform the original maintainer of this.

But I don't think that such a clause is a good idea: very often these internal modifications are not of the required quality. Also, I can easily imagine that some projects might drown in low-quality upstream pushes.

Bug fixes is what I am thinking about.

It is often not clear what is a bug and what is "unexpected/surprising (but 'correct') behavior". Also, many bug fixes of authors that don't have a good understanding of the code cause lots of additional regressions or don't fit the original architecture of the software well.