maint: Add script to backport my pull requests from nixpkgs

Normally, ~/source/nixpkgs follows the releases, but when I contribute to
nixpkgs, it is convenient to have access to my changes in ~/source/nixpkgs
before next release, ideally even before pull request hits the master.

maint/import-commit

@@ -0,0 +1,27 @@
+#!/usr/bin/env python3
+"""
+Import commit from specified local repository into a prefix of this monorepository.
+"""
+import click
+import os
+import tempfile
+from subprocess import check_output, check_call
+
+@click.command()
+@click.option("-s", "--source", default="/home/kaction/devel/nix/nixpkgs")
+@click.option("-P", "--prefix", default="nixpkgs")
+@click.argument("rev")
+def main(source, prefix, rev):
+    with tempfile.TemporaryDirectory() as tempdir:
+        patch_filename = f"{tempdir}/out.patch"
+        here = check_output("git rev-parse --show-toplevel".split(), text=True).strip()
+
+        with open(patch_filename, "w") as fp:
+            os.chdir(source)
+            check_call(["git", "format-patch", "--stdout", f"{rev}~..{rev}"], stdout=fp)
+
+        with open(patch_filename) as fp:
+            os.chdir(here)
+            check_call(["./maint/prefix-patch", prefix], stdin=fp)
+
+main()