sync: properly handle standalone manifests for `sync` command

sync should not attempt to sync the manifest project if it was
created from a standalone manifest. The current work around is to
run sync with --nmu.

BUG=none
TEST=manual runs

Change-Id: I2e121af0badf9642143e77c7af89d1c2d993b0f3
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/321195
Tested-by: Jack Neus <jackneus@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>

subcmds/sync.py

diff --git a/subcmds/sync.py b/subcmds/sync.py
index 7318516..0170409 100644
--- a/subcmds/sync.py
+++ b/subcmds/sync.py
@@ -969,14 +969,16 @@
               file=sys.stderr)
 
     mp = self.manifest.manifestProject
-    mp.PreSync()
+    is_standalone_manifest = mp.config.GetString('manifest.standalone')
+    if not is_standalone_manifest:
+      mp.PreSync()
 
     if opt.repo_upgraded:
       _PostRepoUpgrade(self.manifest, quiet=opt.quiet)
 
     if not opt.mp_update:
       print('Skipping update of local manifest project.')
-    else:
+    elif not is_standalone_manifest:
       self._UpdateManifestProject(opt, mp, manifest_name)
 
     load_local_manifests = not self.manifest.HasLocalManifests