omit inode from ContentIdentifier for directory special remote

Directory special remotes with importtree=yes now avoid unncessary overhead
when inodes of files have changed, as happens whenever a FAT filesystem
gets remounted.

A few unusual edge cases of modifications won't be detected and
imported. I think they're unusual enough not to be a concern. It would
be possible to add a config setting that controls whether to compare
inodes too, but does not seem worth bothering the user about currently.

I chose to continue to use the InodeCache serialization, just with the
inode zeroed. This way, if I later change my mind or make it
configurable, can parse it back to an InodeCache and operate on it. The
overhead of storing a 0 in the content identifier log seems worth it.

There is a one-time cost to this change; all directory special remotes
with importtree=yes will re-hash all files once, and will update the
content identifier logs with zeroed inodes.

This commit was sponsored by Brett Eisenberg on Patreon.

doc/todo/import_tree_from_FAT_does_unncessary_work_due_to_inode_instability.mdwn

@@ -26,3 +26,5 @@ an inode sentinal file and checking it to tell when inodes have changed
 would need importing to write to the drive. That seems strange, and the
 drive could even be read-only. May be the directory special remote should
 just not use inode numbers at all?
+
+> [[done]] --[[Joey]]