fwd_lines.py

#!/usr/bin/env python3

import sys
import difflib
import subprocess

commit = sys.argv[1]
file = sys.argv[2]
line = int(sys.argv[3]) - 1

prev = subprocess.check_output(["git", "show", "%s:%s" % (commit, file)]).decode('utf-8').split("\n")
cur = open(file).read().split("\n")

sm = difflib.SequenceMatcher(None, prev, cur)

for tag, i1, i2, j1, j2 in sm.get_opcodes():
    if i1 <= line < i2:
        if tag != "equal":
            print("line was deleted")
            sys.exit(1)
        corr_line = line - i1 + j1
        break

prev[line] = ">>>" + prev[line] + "<<<"
cur[corr_line] = ">>>" + cur[corr_line] + "<<<"

for i in range(len(prev)):
    prev[i] = str(i+1) + ": " + prev[i]

for i in range(len(cur)):
    cur[i] = str(i+1) + ": " + cur[i]

print("PREV")
print("==================================")
print("\n".join(prev))
print("")

print("CUR, found in", corr_line + 1)
print("==================================")
print("\n".join(cur))
print("")