hghave: cache the result of gethgversion

hghave --test-features calls it 90 times, each one calling hg --version
which takes a tenth of a second on my workstation, adding up to about
10s win on test-hghave.t.

Fixes https://bugs.debian.org/939756

Differential Revision: https://phab.mercurial-scm.org/D8092

tests/hghave.py

@@ -307,13 +307,23 b' def has_lsprof():'
         return False
 
 
-def gethgversion():
+def _gethgversion():
     m = matchoutput('hg --version --quiet 2>&1', br'(\d+)\.(\d+)')
     if not m:
         return (0, 0)
     return (int(m.group(1)), int(m.group(2)))
 
 
+_hgversion = None
+
+
+def gethgversion():
+    global _hgversion
+    if _hgversion is None:
+        _hgversion = _gethgversion()
+    return _hgversion
+
+
 @checkvers(
     "hg", "Mercurial >= %s", list([(1.0 * x) / 10 for x in range(9, 99)])
 )