add annex.dbdir (WIP)

WIP: This is mostly complete, but there is a problem: createDirectoryUnder
throws an error when annex.dbdir is set to outside the git repo.

annex.dbdir is a workaround for filesystems where sqlite does not work,
due to eg, the filesystem not properly supporting locking.

It's intended to be set before initializing the repository. Changing it
in an existing repository can be done, but would be the same as making a
new repository and moving all the annexed objects into it. While the
databases get recreated from the git-annex branch in that situation, any
information that is in the databases but not stored in the branch gets
lost. It may be that no information ever gets stored in the databases
that cannot be reconstructed from the branch, but I have not verified
that.

Sponsored-by: Dartmouth College's Datalad project

Database/ContentIdentifier.hs

@@ -78,7 +78,7 @@ AnnexBranch
  -}
 openDb :: Annex ContentIdentifierHandle
 openDb = do
-	dbdir <- fromRepo gitAnnexContentIdentifierDbDir
+	dbdir <- calcRepo' gitAnnexContentIdentifierDbDir
 	let db = dbdir P.</> "db"
 	ifM (liftIO $ not <$> R.doesPathExist db)
 		( initDb db $ void $