Add mechanism for cron job $Date$ handling

.gitattributes

@@ -0,0 +1 @@
+*.xml filter=dater

expand_date

@@ -0,0 +1,20 @@
+#!/usr/bin/env python3
+
+from os import getcwd
+from subprocess import check_output, DEVNULL
+from sys import stdin, stdout, argv
+
+content = stdin.read()
+
+# To editors:
+# The usage of FETCH_HEAD here is very tricky.  This is only intended for
+# the cron job running "git pull" periodically.  Humans should NOT rely on
+# this script at all.  Do NOT add "dater" filter into your .git/config.
+# For editors, just use `git log ${path}` to show the history of a file.
+cmd = [ 'git', 'log', '-1', '--pretty=%ad', 'FETCH_HEAD', '--', argv[1] ]
+try:
+    date = check_output(cmd, cwd = getcwd(), stderr=DEVNULL).decode().rstrip()
+    content = content.replace('$Date$', '$Date: %s$' % date)
+except Exception:
+    pass
+stdout.write(content)