use disconnected history for import tracking branch

This avoids the first merge from it deleting all files in the current
branch, which was very surpring and unwanted behavior.

Command/Import.hs

@@ -247,10 +247,15 @@ seekRemote :: Remote -> Branch -> Maybe TopFilePath -> CommandSeek
 seekRemote remote branch msubdir = allowConcurrentOutput $ do
 	importtreeconfig <- case msubdir of
 		Nothing -> return ImportTree
-		Just subdir -> frombranch Git.Ref.tree >>= \case
+		Just subdir ->
+			let mk tree = pure $ ImportSubTree subdir tree
+			in fromtrackingbranch Git.Ref.tree >>= \case
+				Just tree -> mk tree
+				Nothing -> inRepo (Git.Ref.tree branch) >>= \case
+					Just tree -> mk tree
 					Nothing -> giveup $ "Unable to find base tree for branch " ++ fromRef branch
-			Just tree -> pure $ ImportSubTree subdir tree
+	
-	parentcommit <- frombranch Git.Ref.sha
+	parentcommit <- fromtrackingbranch Git.Ref.sha
 	let importcommitconfig = ImportCommitConfig parentcommit ManualCommit importmessage
 
 	importable <- download importtreeconfig =<< enumerate
@@ -258,12 +263,10 @@ seekRemote remote branch msubdir = allowConcurrentOutput $ do
 		commitRemote remote branch tb parentcommit importtreeconfig importcommitconfig importable
   where
 	importmessage = "import from " ++ Remote.name remote
+
 	tb = mkRemoteTrackingBranch remote branch
-	-- If the remote tracking branch already exists, get from it,
+
-	-- otherwise get from the branch.
+	fromtrackingbranch a = inRepo $ a (fromRemoteTrackingBranch tb)
-	frombranch a = inRepo (a (fromRemoteTrackingBranch tb)) >>= \case
-		Just v -> return (Just v)
-		Nothing -> inRepo (a branch)
 
 	enumerate = do
 		showStart' "import" (Just (Remote.name remote))

doc/git-annex-export.mdwn

@@ -43,6 +43,9 @@ paragraph do not apply. Note that dropping content from such a remote is
 not supported. See individual special remotes' documentation for
 details of how to enable such versioning.
 
+You can combine using `git annex export` to send changes to a special 
+remote with `git annex import` to fetch changes from a special remote.
+
 The `git annex sync --content` command (and the git-annex assistant)
 can also be used to export a branch to a special remote, 
 updating the special remote whenever the branch is changed.

doc/git-annex-import.mdwn

@@ -24,7 +24,11 @@ This way, something can be using the special remote for file storage,
 adding files, modifying files, and deleting files, and you can track those
 changes using git-annex.
 
-This can only be used with special remotes that were configured with
+You can combine using `git annex import` to fetch changes from a special 
+remote with `git annex export` to send your local changes to the special
+remote.
+
+You can only import from special remotes that were configured with
 `importtree=yes` when set up with [[git-annex-initremote]](1). Only some
 kinds of special remotes will let you configure them this way.
 
@@ -39,7 +43,13 @@ For example:
 	git annex import master --from myremote
 	git merge myremote/master
 
-It's also posible to limit the import to a subdirectory, using the
+Note that you may need to pass `--allow-unrelated-histories` the first time
+you `git merge` from an import. Think of this as the remote being a
+separate git repository with its own files. If you first
+`git annex export` files to a remote, and then `git annex import` from it,
+you won't need that option.
+
+You can also limit the import to a subdirectory, using the
 "branch:subdir" syntax. For example, if "camera" is a special remote
 that accesses a camera, and you want to import those into the photos
 directory, rather than to the root of your repository:
@@ -47,10 +57,6 @@ directory, rather than to the root of your repository:
 	git annex import master:photos --from camera
 	git merge camera/master
 
-You can combine using `git annex import` to fetch changes from a special 
-remote with `git annex export` to send your local changes to the special
-remote.
-
 The `git annex sync --content` command (and the git-annex assistant)
 can also be used to import from a special remote.
 To do this, you need to configure "remote.<name>.annex-tracking-branch"

doc/todo/import_tree.mdwn

@@ -21,24 +21,6 @@ this.
 * export needs to use storeExportWithContentIdentifierM for importtree=yes
   remotes
 
-* Merging the first import from a remote currently removes all local
-  files that are not present in the remote. But subsequent imports from the
-  remote do not delete newly added local files when merged. This seems 
-  inconsistent and very surprising to the user.
-
-  It happens because the remote tracking branch currently starts
-  from the local branch, and then has a commit added to it that contains
-  all files in the remote (and thus deletes the local files).
-  A subsequent import extents the tracking branch by another commit, so
-  merging that does not re-do the deletion.
-
-  One fix would be to start the remote tracking branch not with the local
-  branch. Reflecting it being a perhaps entirely separate line of
-  development. Unless git-annex export were used first, and updated the
-  tracking branch, then the branch would start with what's exported, which
-  is fine. The downside of this is that git merge --allow-unrelated-histories
-  is needed to merge that unrelated history.
-
 * export needs to update the tracking branch with what it exported
 
 * "git annex import master --from rmt" followed by "git annex import master:sub --from rmt"