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

CmdLine/Option.hs

@@ -59,6 +59,11 @@ commonOptions =
 		<> help "show debug messages coming from the specified module"
 		<> hidden
 		)
+	, annexFlag (setexplain True)
+		( long "explain" <> short 'd'
+		<> help "explain why git-annex does what it does"
+		<> hidden
+		)
 	]
   where
 	setforce v = setAnnexRead $ \rd -> rd { Annex.force = v }
@@ -82,5 +87,9 @@ commonOptions =
 			decodeBS (debugfilterconfig <> "=") ++ v
 		]
 	
+	setexplain v = mconcat
+		[ setAnnexRead $ \rd -> rd { Annex.explainenabled = v }
+		]
+	
 	(ConfigKey debugconfig) = annexConfig "debug"
 	(ConfigKey debugfilterconfig) = annexConfig "debugfilter"