importfeed: Fix reversion that caused some '.' in filenames to be replaced with '_'

sanitizeFilePath was changed to sanitize leading '.', but ImportFeed was
running it on parts of the template. So eg the leading '.' in the extension
got sanitized.

Note the added case for sanitizeLeadingFilePathCharacter ('/':_)
-- this was added because, if the template is title/episode and the title
is not set, it would expand to "/episode". So this is another potential
security fix.

Annex/UntrustedFilePath.hs

@@ -31,17 +31,26 @@ import System.FilePath
  - that case.
  -}
 sanitizeFilePath :: String -> FilePath
-sanitizeFilePath [] = "file"
-sanitizeFilePath f = leading (map sanitize f)
+sanitizeFilePath = sanitizeLeadingFilePathCharacter . sanitizeFilePathComponent
+
+{- For when the filepath is being built up out of components that should be
+ - individually sanitized, this can be used for each component, followed by
+ - sanitizeLeadingFilePathCharacter for the whole thing.
+ -}
+sanitizeFilePathComponent :: String -> String
+sanitizeFilePathComponent = map sanitize
   where
 	sanitize c
 		| c == '.' || c == '-' = c
 		| isSpace c || isPunctuation c || isSymbol c || isControl c || c == '/' = '_'
 		| otherwise = c
 
-	leading ('.':s) = '_':s
-	leading ('-':s) = '_':s
-	leading s = s
+sanitizeLeadingFilePathCharacter :: String -> FilePath
+sanitizeLeadingFilePathCharacter [] = "file"
+sanitizeLeadingFilePathCharacter ('.':s) = '_':s
+sanitizeLeadingFilePathCharacter ('-':s) = '_':s
+sanitizeLeadingFilePathCharacter ('/':s) = '_':s
+sanitizeLeadingFilePathCharacter s = s
 
 escapeSequenceInFilePath :: FilePath -> Bool
 escapeSequenceInFilePath f = '\ESC' `elem` f