perf: get subsettable from appropriate module for Mercurial earlier than 2.9 Before this patch, using branchmap.subsettable prevents perfbranchmap from measuring performance of Mercurial earlier than 2.9 (or 175c6fd8cacc), because 175c6fd8cacc moved subsettable from repoview.py to branchmap.py, even though there are some code paths for Mercurial earlier than 2.9 in perf.py. For example, setting "_prereadsize" attribute in perfindex() and perfnodelookup() is effective only with hg earlier than 1.8 (or 61c9bc3da402). To get subsettable from appropriate module, this patch examines existence of subsettable in branchmap and repoview. This patch also adds check-perf-code.py an extra check entry to detect direct usage of subsettable attribute in perf.py.