check symlink before reading file

This is faster because when multiple files are in a directory, it gets
cached.

Annex/Link.hs

@@ -301,7 +301,7 @@ unpaddedMaxPointerSz = 8192
  - symlink does. Avoids a false positive in those cases.
  - -}
 isPointerFile :: RawFilePath -> IO (Maybe Key)
-isPointerFile f = catchDefaultIO Nothing $ do
+isPointerFile f = catchDefaultIO Nothing $
 #if defined(mingw32_HOST_OS)
 	checkcontentfollowssymlinks -- no symlinks supported on windows
 #else
@@ -311,13 +311,10 @@ isPointerFile f = catchDefaultIO Nothing $ do
 		closeFd
 		(\fd -> readhandle =<< fdToHandle fd)
 #else
-	pointercontent <- checkcontentfollowssymlinks
+	ifM (isSymbolicLink <$> R.getSymbolicLinkStatus f)
-	if isJust pointercontent
-		then ifM (isSymbolicLink <$> R.getSymbolicLinkStatus f)
 		( return Nothing
-			, return pointercontent
+		, checkcontentfollowssymlinks
 		)
-		else return Nothing
 #endif
 #endif
   where

doc/bugs/significant_performance_regression_impacting_datal/comment_25_5525b58ec94084a926c71f749d1f4233._comment

@@ -0,0 +1,8 @@
+[[!comment format=mdwn
+ username="joey"
+ subject="""comment 25"""
+ date="2021-06-14T15:52:37Z"
+ content="""
+Found an optimisation that sped it up 50%, but this edge case is still 
+`O(N^2)`, so *shrug*.
+"""]]