pre-commit-annex hook script to automatically extract metadata from lots of types of files

Using the extract(1) program to do the heavy lifting.

Decided to make git-annex run pre-commit-annex when committing. Since
git-annex pre-commit also runs it, it'll be run when git commit is run too,
via the pre-commit hook. This basically gives back the pre-commit hook
that git-annex took away. The implementation avoids repeatedly looking
for the hook script when the assistant is running and committing
repeatedly; only checks if the hook is available once.

To make the script simpler, made git-annex metadata -s field?=value
only set a field when it's not already got a value.

This commit was sponsored by bak.

Types/MetaData.hs

@@ -219,6 +219,7 @@ data ModMeta
 	= AddMeta MetaField MetaValue
 	| DelMeta MetaField MetaValue
 	| SetMeta MetaField MetaValue -- removes any existing values
+	| MaybeSetMeta MetaField MetaValue -- when field has no existing value
 
 {- Applies a ModMeta, generating the new MetaData.
  - Note that the new MetaData does not include all the 
@@ -229,12 +230,16 @@ modMeta _ (DelMeta f oldv) = updateMetaData f (unsetMetaValue oldv) emptyMetaDat
 modMeta m (SetMeta f v) = updateMetaData f v $
 	foldr (updateMetaData f) emptyMetaData $
 		map unsetMetaValue $ S.toList $ currentMetaDataValues f m
+modMeta m (MaybeSetMeta f v)
+	| S.null (currentMetaDataValues f m) = updateMetaData f v emptyMetaData
+	| otherwise = emptyMetaData
 
-{- Parses field=value, field+=value, field-=value -}
+{- Parses field=value, field+=value, field-=value, field?=value -}
 parseModMeta :: String -> Either String ModMeta
 parseModMeta p = case lastMaybe f of
 	Just '+' -> AddMeta <$> mkMetaField f' <*> v
 	Just '-' -> DelMeta <$> mkMetaField f' <*> v
+	Just '?' -> MaybeSetMeta <$> mkMetaField f' <*> v
 	_ -> SetMeta <$> mkMetaField f <*> v
   where
 	(f, sv) = separate (== '=') p