tests/README

merge: move purge logic from extension

Working directory purging feels like functionality that should be
in core rather than in an extension.

This commit effectively moves the core purging logic from the
purge extension to merge.py.

Code was refactored slightly. Rather than deal with printing in
this function, the function is a generator of paths and the caller
can worry about printing.

Differential Revision: https://phab.mercurial-scm.org/D4477

To run the tests, do:

cd tests/
python run-tests.py

See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.