tests: check importing modules in perf.py for historical portability To check importing modules in perf.py for historical portability, this patch lists up files by "hg files" both for "1.2" and tip, and builds up "module whitelist" check from those files. This patch uses "1.2" as earlier side version of "module whitelist", because "mercurial.error" module is a blocker for loading perf.py with Mercurial earlier than 1.2, and just importing "mercurial.error" separately isn't enough.