git_cl: Attempt again to fix xml format on Win

It turns out that we need forward slashes in one place and back
slashes in two other places, so neither path by itself will work.

This CL changes things to do the slash substitution as needed.

Bug: 1502689
Change-Id: I05681ca5aff9f02a1c4ecc14c3c4e877312118ff
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5074657
Reviewed-by: Josip Sokcevic <sokcevic@chromium.org>
Commit-Queue: Dirk Pranke <dpranke@google.com>

git_cl.py

@@ -6349,7 +6349,7 @@ def _FormatXml(opts, paths, top_dir, upstream_commit):
         if not xml_dir:
             continue
 
-        tool_dir = os.path.join(top_dir, xml_dir)
+        tool_dir = os.path.join(top_dir, xml_dir.replace('/', os.path.sep))
         pretty_print_tool = os.path.join(tool_dir, 'pretty_print.py')
         cmd = [shutil.which('vpython3'), pretty_print_tool, '--non-interactive']
 
@@ -6361,12 +6361,12 @@ def _FormatXml(opts, paths, top_dir, upstream_commit):
         # tools/metrics/histogrmas, pretty-print should be run with an
         # additional relative path argument, like: $ python pretty_print.py
         # metadata/UMA/histograms.xml $ python pretty_print.py enums.xml
-        if xml_dir == os.path.join('tools', 'metrics', 'histograms'):
+        if xml_dir == 'tools/metrics/histograms':
             if os.path.basename(path) not in ('histograms.xml', 'enums.xml',
                                               'histogram_suffixes_list.xml'):
                 # Skip this XML file if it's not one of the known types.
                 continue
-            cmd.append(path)
+            cmd.append(path.replace('/', os.path.sep))
 
         if opts.dry_run or opts.diff:
             cmd.append('--diff')