setup: use changes since latest tag instead of just distance

For a Mercurial built on the merge from stable into default right after 3.2.2
was released -- 19ebd2f88fc7 -- the version number produced was "3.2.2+4". This
is potentially misleading, since in reality the built Mercurial includes many
more changes compared to 3.2.2.

Change the versioning scheme so that we take into consideration all the changes
present in the current revision that aren't present in the latest tag. For
19ebd2f88fc7 the new versioning scheme results in a version number of
"3.2.2+256". This gives users a much better idea of how many changes have
actually happened since the latest release.

Since changessincelatesttag is always greater than or equal to the
latesttagdistance, this will produce version numbers that are always greater
than or equal to the old scheme. Thus there's minimal compatibility risk.

setup.py

@@ -196,9 +196,13 b" if os.path.isdir('.hg'):"
         if hgid.endswith('+'): # propagate the dirty status to the tag
             version += '+'
     else: # no tag found
-        cmd = [sys.executable, 'hg', 'parents', '--template',
-               '{latesttag}+{latesttagdistance}-']
-        version = runhg(cmd, env) + hgid
+        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())
+        version = '%s+%s-%s' % (ltag, changessince, hgid)
     if version.endswith('+'):
         version += time.strftime('%Y%m%d')
 elif os.path.exists('.hg_archival.txt'):