re2: exit `_checkre2` early if calling it is useless

If we already know if re2 is supported there is no need to run the check again.
Stricly speaking we don't call it twice, but this seems a simple enough
safeguard to include in this series.

mercurial/util.py

@@ -2214,6 +2214,9 b' class _re:'
     def _checkre2(self):
         global _re2
         global _re2_input
+        if _re2 is not None:
+            # we already have the answer
+            return
 
         check_pattern = br'\[([^\[]+)\]'
         check_input = b'[ui]'