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.

Git/Hook.hs

@@ -15,6 +15,10 @@ data Hook = Hook
 	{ hookName :: FilePath
 	, hookScript :: String
 	}
+	deriving (Ord)
+
+instance Eq Hook where
+	a == b = hookName a == hookName b
 
 hookFile :: Hook -> Repo -> FilePath
 hookFile h r = localGitDir r </> "hooks" </> hookName h