tests: add coverage to ensure Wix tracks 'help' and 'templates' files

This would have caught the problem fixed by 65d2538ac993. There are
other *.wxs files that can be checked, but they appear to be more
complicated. For example, locale.wxs has what appears to be foreach
loop support, as well as variable substitution.

By checking `hg files` to determine tracked file, this is able to avoid false
failures when other junk is present in the filesystem, like *.orig files.

I can't tell if the map-cmdline.status file is not included on purpose, but I
don't see the purpose of excluding it.  The missing help files seem reasonable
for Windows.

tests/test-install.t

@@ -1,43 +1,111 b''
 hg debuginstall
   $ hg debuginstall
   checking encoding (ascii)...
   checking Python executable (*) (glob)
   checking Python version (2.*) (glob)
   checking Python lib (*lib*)... (glob)
   checking installed modules (*mercurial)... (glob)
   checking templates (*mercurial?templates)... (glob)
   checking commit editor...
   checking username...
   no problems detected
 
 hg debuginstall with no username
   $ HGUSER= hg debuginstall
   checking encoding (ascii)...
   checking Python executable (*) (glob)
   checking Python version (2.*) (glob)
   checking Python lib (*lib*)... (glob)
   checking installed modules (*mercurial)... (glob)
   checking templates (*mercurial?templates)... (glob)
   checking commit editor...
   checking username...
    no username supplied
    (specify a username in your configuration file)
   1 problems detected, please check your install!
   [1]
 
 path variables are expanded (~ is the same as $TESTTMP)
   $ mkdir tools
   $ touch tools/testeditor.exe
 #if execbit
   $ chmod 755 tools/testeditor.exe
 #endif
   $ hg debuginstall --config ui.editor=~/tools/testeditor.exe
   checking encoding (ascii)...
   checking Python executable (*) (glob)
   checking Python version (*) (glob)
   checking Python lib (*lib*)... (glob)
   checking installed modules (*mercurial)... (glob)
   checking templates (*mercurial?templates)... (glob)
   checking commit editor...
   checking username...
   no problems detected
+
+  $ cat >> wixxml.py << EOF
+  > import os, subprocess, sys
+  > import xml.etree.ElementTree as ET
+  > 
+  > # MSYS mangles the path if it expands $TESTDIR
+  > testdir = os.environ['TESTDIR']
+  > ns = {'wix' : 'http://schemas.microsoft.com/wix/2006/wi'}
+  > 
+  > def directory(node, relpath):
+  >     '''generator of files in the xml node, rooted at relpath'''
+  >     dirs = node.findall('./wix:Directory', ns)
+  > 
+  >     for d in dirs:
+  >         for subfile in directory(d, relpath + d.attrib['Name'] + '/'):
+  >             yield subfile
+  > 
+  >     files = node.findall('./wix:Component/wix:File', ns)
+  > 
+  >     for f in files:
+  >         yield relpath + f.attrib['Name']
+  > 
+  > def hgdirectory(relpath):
+  >     '''generator of tracked files, rooted at relpath'''
+  >     hgdir = "%s/../mercurial" % (testdir)
+  >     args = ['hg', '--cwd', hgdir, 'files', '--rev', '.', relpath]
+  >     proc = subprocess.Popen(args, stdout=subprocess.PIPE,
+  >                             stderr=subprocess.PIPE)
+  >     output = proc.communicate()[0]
+  > 
+  >     slash = '/'
+  >     for line in output.splitlines():
+  >         if os.name == 'nt':
+  >             yield line.replace(os.sep, slash)
+  >         else:
+  >             yield line
+  > 
+  > tracked = [f for f in hgdirectory(sys.argv[1])]
+  > 
+  > xml = ET.parse("%s/../contrib/wix/%s.wxs" % (testdir, sys.argv[1]))
+  > root = xml.getroot()
+  > dir = root.find('.//wix:DirectoryRef', ns)
+  > 
+  > installed = [f for f in directory(dir, '')]
+  > 
+  > print('Not installed:')
+  > for f in sorted(set(tracked) - set(installed)):
+  >     print('  %s' % f)
+  > 
+  > print('Not tracked:')
+  > for f in sorted(set(installed) - set(tracked)):
+  >     print('  %s' % f)
+  > EOF
+
+  $ python wixxml.py help
+  Not installed:
+    help/common.txt
+    help/hg.1.txt
+    help/hgignore.5.txt
+    help/hgrc.5.txt
+    help/internals/bundles.txt
+    help/internals/changegroups.txt
+  Not tracked:
+
+  $ python wixxml.py templates
+  Not installed:
+    templates/map-cmdline.status
+  Not tracked: