adjust --hide-missing

* At long last there's a way to hide annexed files whose content
  is missing from the working tree: git-annex adjust --hide-missing
* When already in an adjusted branch, running git-annex adjust
  again will update the branch as needed. This is mostly
  useful with --hide-missing to hide/unhide files after their content
  has been dropped or received.

Still needs integration with sync and the assistant, and not as fast as it
could be, but already usable.

This commit was sponsored by Ethan Aubin.

doc/todo/hide_missing_files/comment_4_e3bf8aaecc0f612873609c92814dcd12._comment

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+[[!comment format=mdwn
+ username="joey"
+ subject="""comment 4"""
+ date="2018-10-18T19:27:43Z"
+ content="""
+I implemented `git annex adjust --hide-missing`.
+It can be run after any change to content availability to update
+the adjusted branch, hiding or unhiding files.
+
+My implementation is not as fast as it could be; each update
+is a linear scan of the whole original branch and rebuild of the adjusted
+branch. But it all works so we'll defer speeding this up to later bug
+reports about it being too slow. ;)
+
+What still needs to be done:
+
+* `git annex sync --content` needs to scan the original branch, not the
+  adjusted branch, to find files to transfer.
+* `git annex sync` needs to update the adjusted branch.
+* The assistant also needs to scan the original branch when looking for
+  files to download.
+* The assistant ought to update the adjusted branch at some point after
+  downloads, but it's not clear when. Perhaps this will need to be deferred
+  until it can be done more cheaply, so it can do it after every file.
+"""]]