setup: replace runhg() with an hgcommand helper class

Replace the runhg() function with an hgcommand helper class.  hgcommand has as
run() function similar to runhg(), but no longer requires the caller to pass in
the exact path to python and the hg script, and the environment settings for
invoking hg.

For now this diff contains no behavior changes, but in the future this will
make it easier for the hgcommand helper class to more intelligently figure out
the proper way to invoke hg.

setup.py

@@ -148,23 +148,28 b' def runcmd(cmd, env):'
     out, err = p.communicate()
     return p.returncode, out, err
 
-def runhg(cmd, env):
-    returncode, out, err = runcmd(cmd, env)
-    # If root is executing setup.py, but the repository is owned by
-    # another user (as in "sudo python setup.py install") we will get
-    # trust warnings since the .hg/hgrc file is untrusted. That is
-    # fine, we don't want to load it anyway.  Python may warn about
-    # a missing __init__.py in mercurial/locale, we also ignore that.
-    err = [e for e in err.splitlines()
-           if not e.startswith(b'not trusting file') \
-              and not e.startswith(b'warning: Not importing') \
-              and not e.startswith(b'obsolete feature not enabled')]
-    if err or returncode != 0:
-        printf("stderr from '%s':" % (' '.join(cmd)), file=sys.stderr)
-        printf(b'\n'.join([b'  ' + e for e in err]), file=sys.stderr)
-        return ''
-    return out
+class hgcommand(object):
+    def __init__(self):
+        self.cmd = [sys.executable, 'hg']
+        self.env = gethgenv()
 
+    def run(self, args):
+        cmd = self.cmd + args
+        returncode, out, err = runcmd(cmd, self.env)
+        # If root is executing setup.py, but the repository is owned by
+        # another user (as in "sudo python setup.py install") we will get
+        # trust warnings since the .hg/hgrc file is untrusted. That is
+        # fine, we don't want to load it anyway.  Python may warn about
+        # a missing __init__.py in mercurial/locale, we also ignore that.
+        err = [e for e in err.splitlines()
+               if not e.startswith(b'not trusting file') \
+                  and not e.startswith(b'warning: Not importing') \
+                  and not e.startswith(b'obsolete feature not enabled')]
+        if err or returncode != 0:
+            printf("stderr from '%s':" % (' '.join(cmd)), file=sys.stderr)
+            printf(b'\n'.join([b'  ' + e for e in err]), file=sys.stderr)
+            return ''
+        return out
 
 def gethgenv():
     # Execute hg out of this directory with a custom environment which takes
@@ -180,13 +185,13 b' def gethgenv():'
         # 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()]
-    hgid = runhg([sys.executable, 'hg', 'id', '-i'], env).strip()
+    hg = hgcommand()
+    cmd = ['log', '-r', '.', '--template', '{tags}\n']
+    numerictags = [t for t in hg.run(cmd).split() if t[0:1].isdigit()]
+    hgid = hg.run(['id', '-i']).strip()
     if not hgid:
         # Bail out if hg is having problems interacting with this repository,
         # rather than falling through and producing a bogus version number.
@@ -198,12 +203,10 b" if os.path.isdir('.hg'):"
         if hgid.endswith('+'): # propagate the dirty status to the tag
             version += '+'
     else: # no tag found
-        ltagcmd = [sys.executable, 'hg', 'parents', '--template',
-                   '{latesttag}']
-        ltag = runhg(ltagcmd, env)
-        changessincecmd = [sys.executable, 'hg', 'log', '-T', 'x\n', '-r',
-                           "only(.,'%s')" % ltag]
-        changessince = len(runhg(changessincecmd, env).splitlines())
+        ltagcmd = ['parents', '--template', '{latesttag}']
+        ltag = hg.run(ltagcmd)
+        changessincecmd = ['log', '-T', 'x\n', '-r', "only(.,'%s')" % ltag]
+        changessince = len(hg.run(changessincecmd).splitlines())
         version = '%s+%s-%s' % (ltag, changessince, hgid)
     if version.endswith('+'):
         version += time.strftime('%Y%m%d')
@@ -407,7 +410,7 b' class buildhgextindex(Command):'
         # here no extension enabled, disabled() lists up everything
         code = ('import pprint; from mercurial import extensions; '
                 'pprint.pprint(extensions.disabled())')
-        returncode, out, err = runcmd([sys.executable, '-c', code], env)
+        returncode, out, err = runcmd([sys.executable, '-c', code], gethgenv())
         if err or returncode != 0:
             raise DistutilsExecError(err)