Gracefully ignore bad remove-project line

Sometimes, we don't care if the remove project is referring to a
non-existing project and we can just ignore it.  This change allows us
to ignore remove-project entries if the project that they refer to
doesn't exist, making them effectively a no-op.

Because this change breaks existing configuration, we allow this to be
configuration controlled using the `optional` attribute in the
remove-project tag.

Change-Id: I6313a02983e81344eadcb4e47d7d6b037ee7420e
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/310964
Tested-by: Michael Kelly <mkelly@arista.com>
Reviewed-by: Mike Frysinger <vapier@google.com>
diff --git a/manifest_xml.py b/manifest_xml.py
index ab4be2f..be74bf4 100644
--- a/manifest_xml.py
+++ b/manifest_xml.py
@@ -918,19 +918,19 @@
       if node.nodeName == 'remove-project':
         name = self._reqatt(node, 'name')
 
-        if name not in self._projects:
+        if name in self._projects:
+          for p in self._projects[name]:
+            del self._paths[p.relpath]
+          del self._projects[name]
+
+          # If the manifest removes the hooks project, treat it as if it deleted
+          # the repo-hooks element too.
+          if self._repo_hooks_project and (self._repo_hooks_project.name == name):
+            self._repo_hooks_project = None
+        elif not XmlBool(node, 'optional', False):
           raise ManifestParseError('remove-project element specifies non-existent '
                                    'project: %s' % name)
 
-        for p in self._projects[name]:
-          del self._paths[p.relpath]
-        del self._projects[name]
-
-        # If the manifest removes the hooks project, treat it as if it deleted
-        # the repo-hooks element too.
-        if self._repo_hooks_project and (self._repo_hooks_project.name == name):
-          self._repo_hooks_project = None
-
   def _AddMetaProjectMirror(self, m):
     name = None
     m_url = m.GetRemote(m.remote.name).url