setup: fail if we cannot determine the version number

If running hg fails, exit the setup script unsuccessfully, rather than
proceeding to use a bogus version of "+0-".  Using an invalid version number
causes various tests to fail later.  Failing early makes it easier to identify
the source of the problem.

It is currently easy for setup.py to fail this way since it sets HGRCPTH to the
empty string before running "hg", which may often disable extensions necessary
to interact with the local repository.

setup.py

@@ -184,6 +184,12 b" 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()
+    if not hgid:
+        # Bail out if hg is having problems interacting with this repository,
+        # rather than falling through and producing a bogus version number.
+        # Continuing with an invalid version number will break extensions
+        # that define minimumhgversion.
+        raise SystemExit('Unable to determine hg version from local repository')
     if numerictags: # tag(s) found
         version = numerictags[-1]
         if hgid.endswith('+'): # propagate the dirty status to the tag