This is patchutils, a collection of tools that operate on patch files.

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.

Combinediff generates a single patch from two incremental patches,
allowing you to merge patches together. The resulting patch file only
alters each file once.

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

Fixcvsdiff is for correcting the output of ‘cvs diff’.

Rediff corrects hand-edited patches, by comparing the original patch
with the modified one and adjusting the offsets and counts.

Lsdiff displays a short listing of affected files in a patch file,
along with (optionally) the line numbers of the start of each patch.

Splitdiff separates out patches from a patch file so that each new
patch file only alters any given file once. In this way, a file
containing several incremental patches can be split into individual
incremental patches.

Grepdiff displays a list of the files modified by a patch where the
patch contains a given regular expression.

Recountdiff fixes up counts and offsets in a unified diff.

Unwrapdiff fixes word-wrapped unified diffs.