Makefile: extract script to massage Python scripts

Extract a script that massages Python scripts. This provides a couple of
benefits:

  - The build logic is deduplicated across Make, CMake and Meson.

  - CMake learns to rewrite scripts as-needed at build time instead of
    only writing them at configure time.

Furthermore, we will use this script when introducing Meson to
deduplicate the logic across build systems.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

generate-python.sh

@@ -0,0 +1,20 @@
+#!/bin/sh
+
+set -e
+
+if test $# -ne 3
+then
+	echo >&2 "USAGE: $0 <GIT_BUILD_OPTIONS> <INPUT> <OUTPUT>"
+	exit 1
+fi
+
+GIT_BUILD_OPTIONS="$1"
+INPUT="$2"
+OUTPUT="$3"
+
+. "$GIT_BUILD_OPTIONS"
+
+sed -e "1s|#!.*python|#!$PYTHON_PATH|" \
+    "$INPUT" >"$OUTPUT+"
+chmod a+x "$OUTPUT+"
+mv "$OUTPUT+" "$OUTPUT"