add Database.ContentIdentifier

Does not yet have a way to update with new information from the
git-annex branch, which will be needed when multiple repos are importing
from the same remote.

Annex/Locations.hs

@@ -48,6 +48,8 @@ module Annex.Locations (
 	gitAnnexSmudgeLock,
 	gitAnnexExportDbDir,
 	gitAnnexExportLock,
+	gitAnnexContentIdentifierDbDir,
+	gitAnnexContentIdentifierLock,
 	gitAnnexScheduleState,
 	gitAnnexTransferDir,
 	gitAnnexCredsDir,
@@ -348,6 +350,14 @@ gitAnnexExportDbDir u r = gitAnnexExportDir u r </> "db"
 gitAnnexExportLock :: UUID -> Git.Repo -> FilePath
 gitAnnexExportLock u r = gitAnnexExportDbDir u r ++ ".lck"
 
+{- Directory containing database used to record remote content ids. -}
+gitAnnexContentIdentifierDbDir :: Git.Repo -> FilePath
+gitAnnexContentIdentifierDbDir r = gitAnnexDir r </> "cid"
+
+{- Lock file for writing to the content id database. -}
+gitAnnexContentIdentifierLock :: Git.Repo -> FilePath
+gitAnnexContentIdentifierLock r = gitAnnexContentIdentifierDbDir r ++ ".lck"
+
 {- .git/annex/schedulestate is used to store information about when
  - scheduled jobs were last run. -}
 gitAnnexScheduleState :: Git.Repo -> FilePath