manifest_xml: allow src=. with symlinks

Some Android/Nest manifests are using <linkfile> with src="." to
create stable paths to specific projects.  Allow that specific
use case as it seems reasonable to support.

Bug: https://crbug.com/gerrit/11218
Change-Id: I5eadec257cd58ba0f8687c590ddc250a7a414a85
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/254276
Reviewed-by: Michael Mortensen <mmortensen@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Mike Frysinger <vapier@google.com>

tests/test_manifest_xml.py

@@ -49,6 +49,8 @@ class ManifestValidateFilePaths(unittest.TestCase):
     # We allow symlinks to end in a slash since we allow them to point to dirs
     # in general.  Technically the slash isn't necessary.
     check('foo/', 'bar')
+    # We allow a single '.' to get a reference to the project itself.
+    check('.', 'bar')
 
   def test_bad_paths(self):
     """Make sure bad paths (src & dest) are rejected."""