avoid crash when starting fsck --incremental when one is already running

Turns out sqlite does not like having its database deleted out from
underneath it. It might suffice to empty the table, but I would rather
start each fsck over with a new database, so I added a lock file, and
running incremental fscks use a shared lock.

This leaves one concurrency bug left; running two concurrent fsck --more
will lead to: "SQLite3 returned ErrorBusy while attempting to perform step."
and one or both will fail. This is a concurrent writers problem.

Locations.hs

@@ -58,6 +58,7 @@ module Locations (
 	gitAnnexRemotesDir,
 	gitAnnexAssistantDefaultDir,
 	gitAnnexFsckDb,
+	gitAnnexFsckDbLock,
 	isLinkToAnnex,
 	HashLevels(..),
 	hashDirMixed,
@@ -345,6 +346,10 @@ gitAnnexAssistantDefaultDir = "annex"
 gitAnnexFsckDb :: Git.Repo -> FilePath
 gitAnnexFsckDb r = gitAnnexDir r </> "fsck.db"
 
+{- Lock file for the fsck database. -}
+gitAnnexFsckDbLock :: Git.Repo -> FilePath
+gitAnnexFsckDbLock r = gitAnnexDir r </> "fsck.dbl"
+
 {- Checks a symlink target to see if it appears to point to annexed content.
  -
  - We only look at paths inside the .git directory, and not at the .git