Patchutils contains a collection of tools for manipulating patch files:
interdiff, combinediff, filterdiff, fixcvsdiff, rediff, lsdiff, and
splitdiff. You can use interdiff to create an incremental patch between
two patches that are against a common source tree. Combinediff can be
used for creating a cumulative diff from two incremental patches.
Filterdiff is for extracting or excluding patches from a patch set
based on modified files matching shell wildcards. Lsdiff lists modified
files in a patch. Rediff corrects hand-edited patches.