sync: skip the bloat check for projects without a worktree

Skip shallow clone bloat checks during --network-only syncs and
filter out projects whose worktree directory does not exist on disk.

Also return None quietly from _GetStatusSnapshot() when a project
lacks an existing worktree directory.

Bug: 565047698
Change-Id: Ia4c7db36a2bd78bc89e7b72f7a492b901b50efa7
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/634425
Commit-Queue: Gavin Mak <gavinmak@google.com>
Reviewed-by: Brian Gan <brgan@google.com>
Tested-by: Gavin Mak <gavinmak@google.com>

subcmds/sync.py

@@ -1608,9 +1608,6 @@ later is required to fix a server side protocol bug.
         """
         project = cls.get_parallel_context()["projects"][project_index]
 
-        if not project.Exists or not project.worktree:
-            return None
-
         # Only check dirty or locally modified projects. These can't be
         # freshly cloned and will accumulate garbage.
         try:
@@ -1660,16 +1657,27 @@ later is required to fix a server side protocol bug.
         run 'git count-objects -v' and warn if the repository is accumulating
         excessive pack files or garbage.
         """
+        # --network-only promises not to touch worktrees, but git status and
+        # update-index --refresh can both rewrite the index.
+        if opt.network_only:
+            return
+
         # We only care about bloated projects if we have a git version that
         # supports --no-auto-gc (2.23.0+) since what we use to disable auto-gc
         # in Project._RemoteFetch.
         if not git_require((2, 23, 0)):
             return
 
+        # Skip projects with no worktree on disk, e.g. ones only ever synced
+        # with --network-only.
         projects = [
             p
             for p in projects
-            if p.clone_depth and not p.stateless_prune_needed
+            if p.clone_depth
+            and not p.stateless_prune_needed
+            and p.worktree
+            and p.Exists
+            and platform_utils.isdir(p.worktree)
         ]
         if not projects:
             return