Transmute a git diff of a file into CriticMark markup or raw HTML tags, in case your markdown processor won't accept CriticMark.

diff-critic.py

#!/usr/bin/python

from __future__ import print_function

import subprocess
import os
import re


def criticize(root, from_ref, to_ref, filename, html=False):
    # Git uses "^+" for additions, "^-" for deletions, "^~$" for newlines. It's
    # always deletion first additions second, so when a deletion is followed by
    # addition, it's a change {~~this~>that~~}, otherwise {--deletion--} and
    # {++addition++}.

    # There is probably a simpler way to do this, but this one works
    # passably well.

    git_args = [
        "git", "diff", "--unified=100000", "--word-diff=porcelain",
        "--no-color",
        from_ref, to_ref, filename
    ]

    p = subprocess.Popen(git_args, cwd=root, stdout=subprocess.PIPE,
                         universal_newlines=True)
    p.wait()

    if p.returncode > 0:
        raise IOError("Git complained about something.")

    raw_data = p.stdout.read()

    # Now we massage this into CriticMark format.

    # Cut off the unified diff header.
    raw_data = re.sub(
        r"^diff --git .*\nindex .*\n--- .*\n\+\+\+ .*\n@@ .* @@\n", "",
        raw_data, flags=re.MULTILINE)

    # Replace all cases of changed pairs, i.e. - followed by a +.

    raw_data = re.sub(
        r"^-(?P<from>.*)\n\+(?P<to>.*)\n",
        r" <del>\g<from></del><ins>\g<to></ins>\n" if html
        else r" {~~\g<from>~>\g<to>~~}\n",
        raw_data,
        flags=re.MULTILINE
    )

    # Replace solo deletions and additions.
    raw_data = re.sub(
        r"^-(?P<del>.*)\n",
        r" <del>\g<del></del>\n" if html else r" {--\g<del>--}\n",
        raw_data,
        flags=re.MULTILINE
    )
    raw_data = re.sub(
        r"^\+(?P<add>.*)\n",
        r" <ins>\g<add></ins>\n" if html else r" {++\g<add>++}\n",
        raw_data,
        flags=re.MULTILINE
    )

    # The rest is easier to do manually:

    # If a line starts with a space, cut the space off and join it to the
    # previous one.

    # If a line starts with a tilde, convert it to a straight newline.

    output_data = ""

    for line in raw_data.split('\n'):
        if line.startswith('~'):
            output_data += "\n"
        elif line.startswith(" "):
            output_data += line[1:]
        else:
            output_data += line

    return output_data

if __name__ == "__main__":

    import argparse
    import sys

    def output_pattern(filename):
        if any([filename.endswith(x) for x in ['.md', '.markdown']]):
            return ".".join(filename.split('.')[:-1]) + ".critic.md"
        return filename + ".critic.md"

    parser = argparse.ArgumentParser(
        description="Using git's word-based diff facility, produce a "
        "diff between two versions of a given file expressed as CriticMark "
        "markup.\nAssumes it's running in the root of a git tree.\n"
        "Nothing will be written if the file did not change between the "
        "given revisions."
    )

    parser.add_argument('filename', metavar='FILE', type=str, nargs='+',
                        help='Filenames to produce a diff for.')

    parser.add_argument('--output', '-o', metavar='output_filename', type=str,
                        help="Output filename. Can "
                        "only be used when processing files one by one, "
                        "otherwise default pattern *.critic.md is used.")

    parser.add_argument('--html', action="store_true",
                        help="Output HTML instead of CriticMark.")

    parser.add_argument('--from', '-f', dest='from_ref', metavar='from',
                        type=str, required=True,
                        help="The 'before' version of the file, required.")

    parser.add_argument('--to', '-t', dest='to_ref', metavar='to', type=str,
                        default='HEAD',
                        help="The 'after' version of the file, "
                        "defaults to HEAD.")

    args = parser.parse_args()

    if not os.path.isdir('.git'):
        print("Not running in a git tree. Use -h for help.")
        sys.exit(1)

    if len(args.filename) > 1 and args.output:
        print("Cannot write multiple output files into the same filename.")
        sys.exit(1)

    for filename in args.filename:
        data = criticize(os.getcwd(), args.from_ref, args.to_ref,
                         filename, html=args.html)
        if len(data):
            with open(args.output or output_pattern(filename), 'w+b') as f:
                f.write(data)