initial implementation of --explain

Currently it only displays explanations of options like --in and --copies.

In the future, it should explain preferred content expression evaluation
and other decisions.

The explanations of a few things could be better. In particular,
"standard" will just appear as-is (or as "!standard" if it doesn't
match), rather than explaining why the standard preferred content expression
for the group matches or not.

Currently as implemented, it goes to stdout, and so commands like
git-annex find that have custom output will not display --explain
information. Perhaps that should change, dunno.

Sponsored-by: Dartmouth College's DANDI project

Messages.hs

@@ -50,6 +50,7 @@ module Messages (
 	outputMessage,
 	withMessageState,
 	MessageState,
+	explain,
 	prompt,
 	mkPrompter,
 	sanitizeTopLevelExceptionMessages,
@@ -299,6 +300,14 @@ jsonOutputEnabled = withMessageState $ \s -> return $
 		JSONOutput _ -> True
 		_ -> False
 
+explain :: Maybe RawFilePath -> StringContainingQuotedPath -> Annex ()
+explain Nothing _ = return ()
+explain (Just f) msg = do
+	rd <- Annex.getRead id
+	when (Annex.explainenabled rd) $
+		outputMessage JSON.none id $
+			"[" <> QuotedPath f <> " " <> msg <> "]\n"
+
 {- Prevents any concurrent console access while running an action, so
  - that the action is the only thing using the console, and can eg prompt
  - the user.