make sync update adjusted branch to hide/unhide

This completes initial support for --hide-missing, although the
assistant still needs to be updated and it perhaps needs to be sped up,
and maybe there needs to be a way for git-annex get to operate on
missing files. Opened some more todos for those things.

This commit was sponsored by Henrik Riomar.

doc/todo/better_way_to_get_missing_files.mdwn

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+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?