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