setup: move environment computation into a helper function

Add a helper function to compute the environment used for invoking mercurial,
rather than doing this computation entirely at global scope.  This will make it
easier to do some subsequent refactoring.

setup.py

@@ -165,10 +165,10 b' def runhg(cmd, env):'
         return ''
     return out
 
-version = ''
 
-# Execute hg out of this directory with a custom environment which takes care
-# to not use any hgrc files and do no localization.
+def gethgenv():
+    # Execute hg out of this directory with a custom environment which takes
+    # care to not use any hgrc files and do no localization.
 env = {'HGMODULEPOLICY': 'py',
        'HGRCPATH': '',
        'LANGUAGE': 'C',
@@ -180,6 +180,9 b" if 'SystemRoot' in os.environ:"
     # https://bugs.python.org/issue13524#msg148850
     env['SystemRoot'] = os.environ['SystemRoot']
 
+env = gethgenv()
+version = ''
+
 if os.path.isdir('.hg'):
     cmd = [sys.executable, 'hg', 'log', '-r', '.', '--template', '{tags}\n']
     numerictags = [t for t in runhg(cmd, env).split() if t[0:1].isdigit()]