Here's a cheat to find the appropriate log entries: Install
CVSync, obtain a copy of the relatively small CVSROOT repository, then grep the Changelog* files looking for comments about the hack, which will lead you to the appropriate source.
I don't have access to my CVS repositories today, else I would just do this myself.
BTW, a search of the misc@ archives showed that the hack was in place at least as far back as 3.4 or 3.5. That would be in the XF4 repository, which has been retained for historical purposes such as these. Xenocara was put in place for the restructured Xorg 6.9, if I remember correctly.
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Edit: nope, Xenocara was for X.Org 7