manifest_xml: harmonize list fields

We allow project.groups to be whitespace or comma delimited, but
repo-hooks.enabled-list is only whitespace delimited.  This hasn't
been a big deal as it's only ever had one valid value, but if we
want to add more, we should harmonize these a bit.

Refactor the groups method to be more generic, and run the enabled-
list attribute through it.  Then add missing docs for it.

Change-Id: Iaa96a0faa9c4a68b313b49336751831b73bf855d
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/290743
Reviewed-by: Michael Mortensen <mmortensen@google.com>
Tested-by: Mike Frysinger <vapier@google.com>

tests/test_manifest_xml.py

@@ -212,6 +212,19 @@ class XmlManifestTests(unittest.TestCase):
         '<manifest></manifest>')
     self.assertEqual(manifest.ToDict(), {})
 
+  def test_repo_hooks(self):
+    """Check repo-hooks settings."""
+    manifest = self.getXmlManifest("""
+<manifest>
+  <remote name="test-remote" fetch="http://localhost" />
+  <default remote="test-remote" revision="refs/heads/main" />
+  <project name="repohooks" path="src/repohooks"/>
+  <repo-hooks in-project="repohooks" enabled-list="a, b"/>
+</manifest>
+""")
+    self.assertEqual(manifest.repo_hooks_project.name, 'repohooks')
+    self.assertEqual(manifest.repo_hooks_project.enabled_repo_hooks, ['a', 'b'])
+
   def test_project_group(self):
     """Check project group settings."""
     manifest = self.getXmlManifest("""