pycompat: wrap xrange for py2 to provide efficient __contains__
The C implementation of xrange in Python 2 provides a O(n) membership
test, which is noticable on pull-based clones of large repositories.
Avoid this by providing a wrapper class with O(1) membership test based
on the edges of the range.
Differential Revision: https://phab.mercurial-scm.org/D4313
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