git_cl: Fix xml format on Win

Diff paths are no longer normalized, and we can use forward slashes as
separators regardless of the platform.

R=dpranke

Fixed: 1502689
Change-Id: I75a2fe824972f0d98b28eb2146c6f05ee4ae2d75
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5038641
Commit-Queue: Dirk Pranke <dpranke@google.com>
Auto-Submit: Josip Sokcevic <sokcevic@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@google.com>

git_cl.py

@@ -6521,10 +6521,10 @@ def CMDformat(parser, args):
 
 def GetMetricsDir(diff_xml):
     metrics_xml_dirs = [
-        os.path.join('tools', 'metrics', 'actions'),
-        os.path.join('tools', 'metrics', 'histograms'),
-        os.path.join('tools', 'metrics', 'structured'),
-        os.path.join('tools', 'metrics', 'ukm'),
+        'tools/metrics/actions',
+        'tools/metrics/histograms',
+        'tools/metrics/structured',
+        'tools/metrics/ukm',
     ]
     for xml_dir in metrics_xml_dirs:
         if diff_xml.startswith(xml_dir):