util.re: add an escape method

The escape method in at least one of the modules called 're2' is in C. This
means it is significantly faster than the Python code written in 're'.

An upcoming patch will have benchmarks.

mercurial/util.py

@@ -744,6 +744,21 b' class _re(object):'
                 pass
         return remod.compile(pat, flags)
 
+    @propertycache
+    def escape(self):
+        '''Return the version of escape corresponding to self.compile.
+
+        This is imperfect because whether re2 or re is used for a particular
+        function depends on the flags, etc, but it's the best we can do.
+        '''
+        global _re2
+        if _re2 is None:
+            self._checkre2()
+        if _re2:
+            return re2.escape
+        else:
+            return remod.escape
+
 re = _re()
 
 _fspathcache = {}