allow pathdepth to drop from the front or take from the end (negative)

Command/AddUrl.hs

@@ -90,13 +90,12 @@ url2file :: URI -> Maybe Int -> FilePath
 url2file url pathdepth = case pathdepth of
 	Nothing -> filesize $ escape fullurl
 	Just depth
-		| depth > 0 -> filesize $ join "/" $
-			fromend depth $ map escape $
-			filter (not . null) $ split "/" fullurl
-		| otherwise -> error "bad --pathdepth value"
+		| depth > 0 -> frombits $ drop depth
+		| otherwise -> frombits $ reverse . take (negate depth) . reverse
 	where
 		fullurl = uriRegName auth ++ uriPath url ++ uriQuery url
+		frombits a = filesize $ join "/" $ a urlbits
+		urlbits = map escape $ filter (not . null) $ split "/" fullurl
 		auth = fromMaybe (error $ "bad url " ++ show url) $ uriAuthority url
 		filesize = take 255
 		escape = replace "/" "_" . replace "?" "_"
-		fromend n = reverse . take n . reverse

doc/git-annex.mdwn

@@ -150,9 +150,10 @@ subdirectories).
   To avoid immediately downloading the url, specify --fast.
 
   Normally the filename is based on the full url, so will look like
-  "www.example.com_subdir_bigfile". For a shorter filename, specify
+  "www.example.com_dir_subdir_bigfile". For a shorter filename, specify
   --pathdepth=N. For example, --pathdepth=2 will use "subdir/bigfile",
-  while --parhdepth=1 will use "bigfile".
+  while --pathdepth=3 will use "bigfile". It can also be negative;
+  --pathdepth=-2 will use the last two parts of the url.
 
   Or, to directly specify what file the url is added to, specify --file.
   This changes the behavior; now all the specified urls are recorded as