fix:journal: repair 1.50's journal reading slowness [#2493]

Since 1.50, sourceFilePath, which does IO operations, was being called for every item in the journal.
On my machine this was causing a ~40% slowdown,
but probably it could be more depending on storage system.

Now it's once again called only once per include directive.
Speed seems slightly better now than 1.43 for some reason
(eg: 13k txns/s -> 8k txns/s -> 14k txns/s).

hledger-lib/Hledger/Read/JournalReader.hs

@@ -303,8 +303,7 @@ includedirectivep :: MonadIO m => InputOpts -> ErroringJournalParser m ()
 includedirectivep iopts = do
   -- save the position at start of include directive, for error messages
   eoff <- getOffset
-  -- and the parent file's path, for error messages and debug output
+  pos <- getSourcePos
-  parentf <- getSourcePos >>= sourcePosFilePath
 
   -- parse the directive
   string "include"
@@ -312,6 +311,7 @@ includedirectivep iopts = do
   prefixedglob <- rstrip . T.unpack <$> takeWhileP Nothing (`notElem` [';','\n'])
   lift followingcommentp
   let (mprefix,glb) = splitReaderPrefix prefixedglob
+  parentf <- sourcePosFilePath pos   -- a little slow, don't do too often
   when (null $ dbg6 (parentf <> " include: glob pattern") glb) $
     customFailure $ parseErrorAt eoff $ "include needs a file path or glob pattern argument"
 
@@ -496,6 +496,8 @@ includedirectivep iopts = do
 -- (since the parse file's path is probably always absolute).
 sourcePosFilePath :: (MonadIO m) => SourcePos -> m FilePath
 sourcePosFilePath = liftIO . canonicalizePath . sourceName
+-- "canonicalizePath is a very big hammer. If you only need an absolute path, makeAbsolute is sufficient"
+-- but we only do this once per include directive, seems ok to leave it as is.
 
 -- | Lift an IO action into the exception monad, rethrowing any IO
 -- error with the given message prepended.