remove wrong uniqueness constraint from ContentIdentifier db

Fix bug that caused importing from a special remote to repeatedly download
unchanged files when multiple files in the remote have the same content.

Unfortunately, there's really no good way to remove a uniqueness constraint
from a sqlite database. The best that can be done is to make a new table
and copy the data over. But that would require using persistent's
migrations or raw sql, and I don't want to do either.

Instead, a sledgehammer approach: Renamed .git/annex/cid to
.git/annex/cids. When the new database doesn't exist, it will be populated
from the git-annex branch.

Noting deletes the old database. Don't want to delete it out from under
some long-running git-annex process that might be using it. It could
eventually be deleted. But this is such a new feature, probably few repos
have the database in any case.

Annex/Locations.hs

@@ -355,9 +355,13 @@ gitAnnexExportLock u r = gitAnnexExportDbDir u r ++ ".lck"
 gitAnnexExportUpdateLock :: UUID -> Git.Repo -> FilePath
 gitAnnexExportUpdateLock u r = gitAnnexExportDbDir u r ++ ".upl"
 
-{- Directory containing database used to record remote content ids. -}
+{- Directory containing database used to record remote content ids.
+ -
+ - (This used to be "cid", but a problem with the database caused it to
+ - need to be rebuilt with a new name.)
+ -}
 gitAnnexContentIdentifierDbDir :: Git.Repo -> FilePath
-gitAnnexContentIdentifierDbDir r = gitAnnexDir r </> "cid"
+gitAnnexContentIdentifierDbDir r = gitAnnexDir r </> "cids"
 
 {- Lock file for writing to the content id database. -}
 gitAnnexContentIdentifierLock :: Git.Repo -> FilePath