Avoid unncessary write to the location log when a file is unlocked and then added back with unchanged content.

Implemented with no additional overhead of compares etc.

This is safe to do for presence logs because of their locality of change;
a given repo's presence logs are only ever changed in that repo, or in a
repo that has just been actively changing the content of that repo.

So, we don't need to worry about a split-brain situation where there'd
be disagreement about the location of a key in a repo. And so, it's ok to
not update the timestamp when that's the only change that would be made
due to logging presence info.

Logs/Presence.hs

@@ -4,7 +4,7 @@
  - a way that can be union merged.
  -
  - A line of the log will look like: "date N INFO"
- - Where N=1 when the INFO is present, and 0 otherwise.
+ - Where N=1 when the INFO is present, 0 otherwise.
  - 
  - Copyright 2010-2014 Joey Hess <id@joeyh.name>
  -
@@ -14,6 +14,7 @@
 module Logs.Presence (
 	module X,
 	addLog,
+	maybeAddLog,
 	readLog,
 	logNow,
 	currentLog,
@@ -28,10 +29,21 @@ import Common.Annex
 import qualified Annex.Branch
 import Git.Types (RefDate)
 
+{- Adds a LogLine to the log, removing any LogLines that are obsoleted by
+ - adding it. -}
 addLog :: FilePath -> LogLine -> Annex ()
-addLog file line = Annex.Branch.change file $ \s -> 
+addLog file line = Annex.Branch.change file $ \s ->
 	showLog $ compactLog (line : parseLog s)
 
+{- When a LogLine already exists with the same status and info, but an
+ - older timestamp, that LogLine is preserved, rather than updating the log
+ - with a newer timestamp.
+ -}
+maybeAddLog :: FilePath -> LogLine -> Annex ()
+maybeAddLog file line = Annex.Branch.maybeChange file $ \s -> do
+	m <- insertNewStatus line $ logMap $ parseLog s
+	return $ showLog $ mapLog m
+
 {- Reads a log file.
  - Note that the LogLines returned may be in any order. -}
 readLog :: FilePath -> Annex [LogLine]