implement removeExportDirectory

Not yet called by Command.Export.

WebDAV needs this to clean up empty collections. Also, example.sh turned
out to not be cleaning up directories when removing content
from them, so it made sense for it to use this.

Remote.Directory did not need it, and since its cleanup method for empty
directories is more efficient than what Command.Export will need to do
to find empty directories, it uses Nothing so that extra work can be
avoided.

This commit was sponsored by Thom May on Patreon.

Types/Remote.hs

@@ -19,6 +19,7 @@ module Types.Remote
 	, Verification(..)
 	, unVerified
 	, ExportLocation(..)
+	, ExportDirectory(..)
 	, isExportSupported
 	, ExportActions(..)
 	)
@@ -164,6 +165,9 @@ unVerified a = do
 newtype ExportLocation = ExportLocation FilePath
 	deriving (Show, Eq)
 
+newtype ExportDirectory = ExportDirectory FilePath
+	deriving (Show, Eq)
+
 isExportSupported :: RemoteA a -> a Bool
 isExportSupported r = exportSupported (remotetype r) (config r) (gitconfig r)
 
@@ -178,6 +182,13 @@ data ExportActions a = ExportActions
 	, retrieveExport :: Key -> ExportLocation -> FilePath -> MeterUpdate -> a Bool
 	-- Removes an exported file (succeeds if the contents are not present)
 	, removeExport :: Key -> ExportLocation -> a Bool
+	-- Removes an exported directory. Typically the directory will be
+	-- empty, but it could possbly contain files or other directories,
+	-- and it's ok to delete those. If the remote does not use
+	-- directories, or automatically cleans up empty directories,
+	-- this can be Nothing. Should not fail if the directory was
+	-- already removed.
+	, removeExportDirectory :: Maybe (ExportDirectory -> a Bool)
 	-- Checks if anything is exported to the remote at the specified
 	-- ExportLocation.
 	-- Throws an exception if the remote cannot be accessed.