add back non-preferred files to imported tree

Prevents merging the import from deleting the non-preferred files from
the branch it's merged into.

adjustTree previously appended the new list of items to the old, which
could result in it generating a tree with multiple files with the same
name. That is not good and confuses some parts of git. Gave it a
function to resolve such conflicts.

That allowed dealing with the problem of what happens when the import
contains some files (or subtrees) with the same name as files that were
filtered out of the export. The files from the import win.

Annex/Locations.hs

@@ -50,6 +50,7 @@ module Annex.Locations (
 	gitAnnexExportDbDir,
 	gitAnnexExportLock,
 	gitAnnexExportUpdateLock,
+	gitAnnexExportExcludeLog,
 	gitAnnexContentIdentifierDbDir,
 	gitAnnexContentIdentifierLock,
 	gitAnnexScheduleState,
@@ -361,6 +362,11 @@ gitAnnexExportLock u r = gitAnnexExportDbDir u r ++ ".lck"
 gitAnnexExportUpdateLock :: UUID -> Git.Repo -> FilePath
 gitAnnexExportUpdateLock u r = gitAnnexExportDbDir u r ++ ".upl"
 
+{- Log file used to keep track of files that were in the tree exported to a
+ - remote, but were excluded by its preferred content settings. -}
+gitAnnexExportExcludeLog :: UUID -> Git.Repo -> FilePath
+gitAnnexExportExcludeLog u r = gitAnnexDir r </> "export.ex" </> fromUUID u
+
 {- Directory containing database used to record remote content ids.
  -
  - (This used to be "cid", but a problem with the database caused it to