merge changes made on other repos into ExportTree

Now when one repository has exported a tree, another repository can get
files from the export, after syncing.

There's a bug: While the database update works, somehow the database on
disk does not get updated, and so the database update is run the next
time, etc. Wasn't able to figure out why yet.

This commit was sponsored by Ole-Morten Duesund on Patreon.

Annex/Locations.hs

@@ -303,7 +303,7 @@ gitAnnexExportDbDir u r = gitAnnexExportDir u r </> "db"
 
 {- Lock file for export state for a special remote. -}
 gitAnnexExportLock :: UUID -> Git.Repo -> FilePath
-gitAnnexExportLock u r = gitAnnexExportDir u r ++ ".lck"
+gitAnnexExportLock u r = gitAnnexExportDbDir u r ++ ".lck"
 
 {- .git/annex/schedulestate is used to store information about when
  - scheduled jobs were last run. -}