ssb: Look back off of readback_time

I manually verified that git log handles this date format. For now,
that's all the testing we really need.

Author: rye

scripts/should-skip-build

@@ -129,7 +129,8 @@ end
 def anything_changed_yesterday?
 	head_time = Commit.HEAD.time
 	readback_time = head_time - 26.hours
-  yesterdays_changes = changes_after('26 hours ago').unique_lines
+
+  yesterdays_changes = changes_after(readback_time.iso8601).unique_lines
 
   yesterdays_changes.count > 0
 end