sync: keep smart sync override with -l

Splitting a sync into `repo sync -n` followed by `repo sync -l` should
end up in the same state as a single `repo sync`. With smart sync it
does not always: -n fetches the revisions pinned by the smart sync
manifest and saves it as smart_sync_override.xml, but the following -l
deletes that override and checks out ToT instead, unless the manifest
sets sync-smartsync="true". So `repo sync -s -n` + `repo sync -l` lands
on ToT, while `repo sync -s` lands on the pinned revisions.

Without an explicit -s or -t, -l does not contact the manifest server,
so it cannot choose new pins itself; the only consistent choice is to
keep the override the previous -n left behind. Do that regardless of
the manifest default.

Syncs that choose a manifest themselves still replace or remove the
override as before: any sync without -l, and -l combined with -s, -t,
-m, --superproject-revision or --no-smart-sync. Smart sync stays
non-sticky for regular syncs, and `repo sync -l --no-smart-sync` still
discards the override to check out ToT.

Bug: 569927750
Change-Id: Id06ff88e9f3183fae32e993e86630fc3e8de71a1
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/640965
Commit-Queue: Rahul Yadav <yadavrah@google.com>
Tested-by: Rahul Yadav <yadavrah@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>

subcmds/sync.py

@@ -2499,17 +2499,18 @@ later is required to fix a server side protocol bug.
         if opt.clone_bundle is None:
             opt.clone_bundle = manifest.CloneBundle
 
-        # An onboarded client run with -l keeps the smart sync manifest it last
-        # synced to instead of contacting the manifest server. Computed before
-        # _ResolveSmartSyncOption, which makes an unset opt.smart_sync
-        # indistinguishable from --no-smart-sync.
+        # Without an explicit smart sync option, -l does not contact the
+        # manifest server, so it keeps the smart sync manifest the client last
+        # synced to, whether that came from -s or from the manifest's
+        # sync-smartsync default. This keeps `sync -n` + `sync -l` equivalent
+        # to a single sync. Computed before _ResolveSmartSyncOption, which
+        # makes an unset opt.smart_sync indistinguishable from --no-smart-sync.
         reuse_smart_sync_override = (
             opt.local_only
             and opt.smart_sync is None
             and not opt.smart_tag
             and not opt.manifest_name
             and not opt.superproject_revision
-            and getattr(manifest.default, "sync_smartsync", False) is True
             and os.path.isfile(smart_sync_manifest_path)
         )