stringutil: add a new function to do minimal regex escaping
Per https://bugs.python.org/
issue29995, re.escape() used to
over-escape regular expression strings, but in Python 3.7 that's been
fixed, which also improved the performance of re.escape(). Since it's
both an output change for us *and* a perfomance win, let's just
effectively backport the new behavior to hg on all Python versions.
Differential Revision: https://phab.mercurial-scm.org/D3841
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