decided not to include export/import trees

They're only needed to cover a gc edge case, and it's better someone
gets caught by that edge case than that someone who does not know about
them ends up with a filtered git-annex branch that contains such a tree
when some of the files listed in it are ones they wanted to *remove*
from the repository.

Command/FilterBranch.hs

@@ -189,7 +189,6 @@ seek o = withOtherTmp $ \tmpdir -> do
 	cmode <- annexCommitMode <$> Annex.getGitConfig
 	cmessage <- Annex.Branch.commitMessage
 	c <- inRepo $ Git.commitTree cmode cmessage [] t
-	-- TODO export.log trees
 	liftIO $ putStrLn (fromRef c)
   where
 	ww = WarnUnmatchLsFiles

doc/git-annex-filter-branch.mdwn

@@ -25,7 +25,7 @@ With no options, no information at all will be included from the git-annex
 branch. Use options to specify what to include. All options can be specified
 multiple times.
 
-Note that, when the repository contains information about a private
+When the repository contains information about a private
 repository (due to `annex.private` being set, or `git-annex initremote
 --private` being used), that private information will be included when
 allowed by the options, even though it is not recorded on the git-annex
@@ -35,6 +35,14 @@ When a repository was created with `git annex initremote --sameas=foo`,
 its information will be included when the information for foo is,
 and excluded when foo is excluded.
 
+When a special remote is configured with importtree=yes or exporttree=yes,
+normally the git tree corresponding to the repository is included in
+the git-annex branch, to make sure it does not get garbage collected
+by `git gc`. Those trees are *not* included when filtering the git-annex
+branch. Usually this will not cause any problems, but if such a tree does
+get garbage collected, it will prevent accessing files on the special
+remote, until the next time a tree is imported or exported to it.
+
 # OPTIONS
 
 * `path`