cache one more log file for metadata

My worry was that a preferred content expression that matches on metadata
would have removed the location log from cache, causing an expensive
re-read when a Seek action later checked the location log.

Especially when the --all optimisation in the previous commit
pre-cached the location log.

This also means that the --all optimisation could cache the metadata log
too, if it wanted too, but not currently done.

The cache is a list, with the most recently accessed file first. That
optimises it for the common case of reading the same file twice, eg a
get, examine, followed by set reads it twice. And sync --content reads the
location log 3 times in a row commonly.

But, as a list, it should not be made to be too long. I thought about
expanding it to 5 items, but that seemed unlikely to be a win commonly
enough to outweigh the extra time spent checking the cache.

Clearly there could be some further benchmarking and tuning here.

Logs.hs

@@ -38,6 +38,26 @@ getLogVariety config f
 	| isMetaDataLog f || f `elem` otherLogs = Just OtherLog
 	| otherwise = PresenceLog <$> firstJust (presenceLogs config f)
 
+{- Typical number of log files that may be read while processing a single
+ - key. This is used to size a cache.
+ -
+ - The location log is generally read, and the metadata log is read when
+ - matching a preferred content expression that matches on metadata,
+ - or when using metadata options.
+ -
+ - When using a remote, the url log, chunk log, remote state log, remote
+ - metadata log, and remote content identifier log might each be used,
+ - but probably at most 3 out of the 6. However, caching too much slows
+ - down all operations because the cache is a linear list, so the cache
+ - is not currently sized to include these.
+ -
+ - The result is that when seeking for files to operate on,
+ - the location log will stay in the cache if the metadata log is also
+ - read.
+ -}
+logFilesToCache :: Int
+logFilesToCache = 2
+
 {- All the old-format uuid-based logs stored in the top of the git-annex branch. -}
 topLevelOldUUIDBasedLogs :: [RawFilePath]
 topLevelOldUUIDBasedLogs =
@@ -59,7 +79,6 @@ topLevelNewUUIDBasedLogs =
 	[ exportLog
 	]
 
-
 {- All the ways to get a key from a presence log file -}
 presenceLogs :: GitConfig -> RawFilePath -> [Maybe Key]
 presenceLogs config f =