Patchutils is a small collection of programs that operate on patch files.
Currently it contains interdiff and filterdiff. 

Interdiff generates an incremental patch from two patches against a common
source. For example, if you have applied a pre-patch to a source tree, and
wish to apply another pre-patch (which is against the same original source
tree), you can use interdiff to generate the patch that you need to apply.
You can also use this to review changes between two pre-patches. 

Filterdiff will select the portions of a patch file that apply to files
matching (or, alternatively, not matching) a shell wildcard. 

WWW: http://cyberelk.net/tim/patchutils/