sync --content now supports --hide-missing adjusted branches

This relies on git ls-files --with-tree, which I'm using in a way that
its man page does not document. Hm. I emailed the git list to try to get
the docs improved, but at least the git test suite does test the same
kind of use case I'm using here.

Performance impact when not in an adjusted branch is limited to some
additional MVar accesses, and a single git call to determine the name of
the current branch. So very minimal.

When in an adjusted branch, the performance impact is
in Annex.WorkTree.lookupFile, which starts doing an equal amount of work
for files that didn't exist as it already did for files that were
unlocked.

This commit was sponsored by Jochen Bartl on Patreon.

doc/todo/hide_missing_files/comment_4_e3bf8aaecc0f612873609c92814dcd12._comment

@@ -14,8 +14,6 @@ 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.