Currently in a `git annex adjust --hide-missing` branch, the only way to
get missing files is to run `git annex sync --content`, with appropriate preferred
content settings or with `-C path` to get a specific path. This is kind of
klunky for ad-hoc getting files.

It would be possible to make `git annex get`, `git annex copy --from remote`, etc
operate on hidden files. Some of the infrastructure is there; it may be as
simple as using LsFiles.inRepoOrBranch instead of LsFiles.inRepo to seek
the files.

Two open questions:

* Would it make more sense for this to be the default behavior when in the
  adjusted branch, or for it to be an option, like "--branch=master"?

  One advantage to the latter is that it would also be usable in bare
  repos, or if you just for some reason want to get files present in some
  other branch than the current one.

  The former might be easier, but it could also be kind of surprising.
  `git annex get .` would download tons of files, which `ls` doesn't
  display.

  Of course, both things could be supported too.

* After getting the files, they'll still be hidden until the branch is
  updated to include them. When should that happen?

  The current method of re-running `git annex adjust --hide-missing` will
  expose them, but the way that works is probably too slow to run after
  every command. Even if it were fast, it would generate a lot of dangling
  objects over time. So leave the update up to the user to run the command
  when they want it? But then the user may get confused, why did it
  download files and they didn't appear?