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.