read a consistent amount from pointer file

A few places were reading the max symlink size of a pointer file,
then passing tp parseLinkTargetOrPointer. Which is fine currently, but
to support pointer files with lines of data after the pointer, enough
has to be read that parseLinkTargetOrPointer can be assured of seeing
enough of that data to know if it's correctly formatted.

Sponsored-by: Dartmouth College's Datalad project

Annex/Link.hs

@@ -82,10 +82,10 @@ getAnnexLinkTarget' file coresymlinks = if coresymlinks
 	probesymlink = R.readSymbolicLink file
 
 	probefilecontent = withFile (fromRawFilePath file) ReadMode $ \h -> do
-		s <- S.hGet h unpaddedMaxPointerSz
+		s <- S.hGet h maxSymlinkSz
 		-- If we got the full amount, the file is too large
 		-- to be a symlink target.
-		return $ if S.length s == unpaddedMaxPointerSz
+		return $ if S.length s == maxSymlinkSz
 			then mempty
 			else 
 				-- If there are any NUL or newline
@@ -331,13 +331,13 @@ formatPointer k = prefix <> keyFile k <> nl
  - memory when reading files that may be pointers.
  -
  - 8192 bytes is plenty for a pointer to a key. This adds some additional
- - padding to allow for any pointer files that might have
- - lines after the key explaining what the file is used for. -}
-maxPointerSz :: Integer
+ - padding to allow for pointer files that have lines of additional data
+ - after the key. -}
+maxPointerSz :: Int
 maxPointerSz = 81920
 
-unpaddedMaxPointerSz :: Int
-unpaddedMaxPointerSz = 8192
+maxSymlinkSz :: Int
+maxSymlinkSz = 8192
 
 {- Checks if a worktree file is a pointer to a key.
  -
@@ -367,7 +367,7 @@ isPointerFile f = catchDefaultIO Nothing $
   where
 	checkcontentfollowssymlinks = 
 		withFile (fromRawFilePath f) ReadMode readhandle
-	readhandle h = parseLinkTargetOrPointer <$> S.hGet h unpaddedMaxPointerSz
+	readhandle h = parseLinkTargetOrPointer <$> S.hGet h maxPointerSz
 
 {- Checks a symlink target or pointer file first line to see if it
  - appears to point to annexed content.