Goal for today was to make `git annex sync --content` operate on files hidden by
`git annex adjust --hide-missing`. However, this got into the weeds pretty
quickly due to the problem of how to handle `--content-of=path` when
either the whole path or some files within it may be hidden.

Eventually I discovered that `git ls-files --with-tree` can be used to
get a combined list of files in the index plus files in another tree,
which in git-annex's case is the original branch that got adjusted.
It's not documented to work the way I'm using it (worrying), but it's
perfect, because git-annex already uses `git ls-files` extensively and this
could let lots of commands get support for operating on hidden files.

That said, I'm going to limit it to `git annex sync` for now,
because it would be a lot of work to make lots of commands support them,
and there could easily be commands where supporting them adds lots of
complexity or room for confusion.

Demo time:

	joey@darkstar:/tmp> git clone ~/lib/sound/
	Cloning into 'sound'...
	done.
	Checking out files: 100% (45727/45727), done.
	joey@darkstar:/tmp> cd sound/
	joey@darkstar:/tmp/sound> git annex init --version=6
	init  (merging origin/git-annex origin/synced/git-annex into git-annex...)
	(scanning for unlocked files...)
	ok
	joey@darkstar:/tmp/sound> git annex adjust --hide-missing
	adjust 
	Switched to branch 'adjusted/master(hidemissing)'
	ok
	joey@darkstar:/tmp/sound#master(hidemissing)> ls
	podcasts
	joey@darkstar:/tmp/sound#master(hidemissing)> ls podcasts
	feeds
	joey@darkstar:/tmp/sound#master(hidemissing)> git annex sync origin --no-push -C podcasts
	...
	joey@darkstar:/tmp/sound> time git annex adjust --hide-missing
	adjust
	ok
	15.03user 3.11system 0:14.95elapsed 121%CPU (0avgtext+0avgdata 93280maxresident)k
	0inputs+88outputs (0major+12206minor)pagefaults 0swaps
	joey@darkstar:/tmp/sound#master(hidemissing)> ls podcasts
	Astronomy_Cast/                                     Hacking_Culture/
	Benjamen_Walker_s_Theory_of_Everything/             In_Our_Time/
	Clarkesworld_Magazine___Science_Fiction___Fantasy/  Lightspeed_MagazineLightspeed_Magazine___Science_Fiction___Fantasy/
	DatCast/                                            Long_Now__Seminars_About_Long_term_Thinking/
	Escape_Pod/                                         Love___Radio/
	Gravy/                                              feeds

Close to being able to use this on my phone. ;-)