metadata: add --tag and --untag shorthand options

Command/MetaData.hs

@@ -17,16 +17,28 @@ import qualified Data.Set as S
 import Data.Time.Clock.POSIX
 
 def :: [Command]
-def = [withOptions [setOption] $ command "metadata" paramPaths seek
+def = [withOptions [setOption, tagOption, untagOption] $
+	command "metadata" paramPaths seek
 	SectionMetaData "sets metadata of a file"]
 
+storeModMeta :: ModMeta -> Annex ()
+storeModMeta modmeta = Annex.changeState $
+	\s -> s { Annex.modmeta = modmeta:Annex.modmeta s }
+
 setOption :: Option
 setOption = Option ['s'] ["set"] (ReqArg mkmod "FIELD[+-]=VALUE") "set metadata"
   where
-	mkmod p = case parseModMeta p of
+	mkmod = either error storeModMeta . parseModMeta
-		Left e -> error e
+
-		Right modmeta -> Annex.changeState $
+tagOption :: Option
-			\s -> s { Annex.modmeta = modmeta:Annex.modmeta s }
+tagOption = Option ['t'] ["tag"] (ReqArg mkmod "TAG") "set a tag"
+  where
+	mkmod = storeModMeta . AddMeta tagMetaField . toMetaValue
+
+untagOption :: Option
+untagOption = Option ['u'] ["untag"] (ReqArg mkmod "TAG") "remove a tag"
+  where
+	mkmod = storeModMeta . AddMeta tagMetaField . mkMetaValue (CurrentlySet False)
 
 seek :: CommandSeek
 seek ps = do

doc/git-annex.mdwn

@@ -708,9 +708,11 @@ subdirectories).
 
   To remove a value, use -s field-=value.
 
-  For example, to set some tags on a file:
+  To set a tag, use -t tag, and use -u tag to remove a tag.
 
-        	git annex metadata annexscreencast.ogv -s tag+=video -s tag+=screencast
+  For example, to set some tags on a file and also its author:
+
+        	git annex metadata annexscreencast.ogv -t video -t screencast -s author+=Alice
 
 * `view [field=value ...] [tag ...]`