preserve unlocked file mtime when dropping

When dropping an unlocked file, preserve its mtime, which avoids git status
unncessarily running the clean filter on the file.

If the index file has close to the same mtime as a work tree file, git will
not trust the index to be up-to-date, and re-runs the clean filter
unncessarily. Preserving the mtime when depopulating a pointer file avoids
git status doing a little (or maybe a lot) of unncessary work.

There are other places that the mtime could be preserved, including other
places where pointer files are written perhaps, but also
populatePointerFile. But, I don't know of cases where those lead to git
status doing unncessary work, so I just fixed the one I'm aware of for now.

Annex/Content/PointerFile.hs

@@ -9,15 +9,20 @@
 
 module Annex.Content.PointerFile where
 
+#if ! defined(mingw32_HOST_OS)
+import System.Posix.Files
+#else
 import System.PosixCompat.Files
+#endif
 
 import Annex.Common
 import Annex.Perms
 import Annex.Link
 import Annex.ReplaceFile
 import Annex.InodeSentinal
-import Utility.InodeCache
 import Annex.Content.LowLevel
+import Utility.InodeCache
+import Utility.Touch
 
 {- Populates a pointer file with the content of a key. 
  -
@@ -48,10 +53,18 @@ populatePointerFile restage k obj f = go =<< liftIO (isPointerFile f)
  - Does not check if the pointer file is modified. -}
 depopulatePointerFile :: Key -> FilePath -> Annex ()
 depopulatePointerFile key file = do
-	mode <- liftIO $ catchMaybeIO $ fileMode <$> getFileStatus file
+	st <- liftIO $ catchMaybeIO $ getFileStatus file
+	let mode = fmap fileMode st
 	secureErase file
 	liftIO $ nukeFile file
 	ic <- replaceFile file $ \tmp -> do
 		liftIO $ writePointerFile tmp key mode
+#if ! defined(mingw32_HOST_OS)
+		-- Don't advance mtime; this avoids unncessary re-smudging
+		-- by git in some cases.
+		liftIO $ maybe noop
+			(\t -> touch tmp t False)
+			(fmap modificationTimeHiRes st)
+#endif
 		withTSDelta (liftIO . genInodeCache tmp)
 	maybe noop (restagePointerFile (Restage True) file) ic