graft exported tree into git-annex branch

So it will be available later and elsewhere, even after GC.

I first though to use git update-index to do this, but feeding it a line
with a tree object seems to always cause it to generate a git subtree
merge. So, fell back to using the Git.Tree interface to maniupulate the
trees, and not involving the git-annex branch index file at all.

This commit was sponsored by Andreas Karlsson.

Command/Export.hs

@@ -70,7 +70,9 @@ seek :: ExportOptions -> CommandSeek
 seek o = do
 	r <- getParsed (exportRemote o)
 	new <- fromMaybe (error "unknown tree") <$>
-		inRepo (Git.Ref.sha (exportTreeish o))
+		-- Dereference the tree pointed to by the branch, commit,
+		-- or tag.
+		inRepo (Git.Ref.tree (exportTreeish o))
 	old <- getExport (uuid r)
 
 	when (length old > 1) $