Makefile: extract script to generate gitweb.js

Similar to the preceding commit, also extract the script to generate the
"gitweb.js" file. While the logic itself is trivial, it helps us avoid
duplication of logic across build systems and ensures that the build
systems will remain in sync with each other in case the logic ever needs
to change.

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

gitweb/generate-gitweb-js.sh

@@ -0,0 +1,12 @@
+#!/bin/sh
+
+if test "$#" -lt 2
+then
+	echo >&2 "USAGE: $0 <OUTPUT> <INPUT>..."
+	exit 1
+fi
+
+OUTPUT="$1"
+shift
+
+cat "$@" >"$OUTPUT"