import: make --dry output valid journal format

[ci skip]

hledger/Hledger/Cli/Commands/Import.hs

@@ -51,7 +51,7 @@ importcmd opts@CliOpts{rawopts_=rawopts,inputopts_=iopts} j = do
           case sortBy (comparing tdate) $ jtxns newj of
             [] -> putStrLn "no new transactions"
             newts | dryrun -> do
-              printf "would import %d new transactions:\n\n" (length newts)
+              printf "; would import %d new transactions:\n\n" (length newts)
               -- TODO how to force output here ?
               -- length (jtxns newj) `seq` print' opts{rawopts_=("explicit",""):rawopts} newj
               mapM_ (putStr . showTransactionUnelided) newts

hledger/doc/commands.m4.md

@@ -409,6 +409,12 @@ New transactions are detected in the same way as print --new:
 by assuming transactions are always added to the input files in increasing date order,
 and by saving `.latest.FILE` state files.
 
+The --dry-run output is in journal format, so you can filter it, eg 
+to see only uncategorised transactions: 
+```shell
+$ hledger import --dry ... | hledger -f- print unknown --ignore-assertions
+```
+
 ## incomestatement
 Show an income statement. Alias: is.