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/git-annex-adjust.mdwn

@@ -54,11 +54,15 @@ This command can only be used in a v6 git-annex repository.
   changes, so if you `git annex drop` files, they will become broken
   links in the usual way. And when files that were missing are copied into the
   repository from elsewhere, they won't immediatly become visible in the
-  branch. 
+  branch.
   
   To update the adjusted branch to reflect changes to content availability, 
   run `git annex adjust --hide-missing` again.
 
+  Despite missing files being hidden, `git annex sync --content` will
+  still operate on them, and can be used to download missing
+  files from remotes.
+
   This option can be combined with --unlock or --fix.
 
 # SEE ALSO

doc/git-annex-sync.mdwn

@@ -70,15 +70,15 @@ by running "git annex sync" on the remote.
 * `--content`, `--no-content`
 
   Normally, syncing does not transfer the contents of annexed files.
-  The --content option causes the content of files in the work tree
+  The --content option causes the content of annexed files
   to also be uploaded and downloaded as necessary.
 
   The `annex.synccontent` configuration can be set to true to make content
   be synced by default.
 
-  Normally this tries to get each annexed file in the work tree
-  that the local repository  does not yet have, and then copies each
-  file in the work tree to every remote that it is syncing with.
+  Normally this tries to get each annexed file that the local repository
+  does not yet have, and then copies each file to every remote that it
+  is syncing with.
   This behavior can be overridden by configuring the preferred content
   of a repository. See [[git-annex-preferred-content]](1).
 
@@ -88,7 +88,7 @@ by running "git annex sync" on the remote.
 
 * `--content-of=path` `-C path`
 
-  While --content operates on all annexed files in the work tree,
+  While --content operates on all annexed files,
   --content-of allows limiting the transferred files to ones in a given
   location.
 

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.