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.

Command/Sync.hs

@@ -18,6 +18,7 @@ import qualified Types.Remote as Remote
 import Annex.Direct
 import Annex.CatFile
 import Annex.Link
+import Annex.Hook
 import qualified Git.Command
 import qualified Git.LsFiles as LsFiles
 import qualified Git.Merge
@@ -156,6 +157,7 @@ commitStaged commitmessage = go =<< inRepo Git.Branch.currentUnsafe
   where
 	go Nothing = return False
 	go (Just branch) = do
+		runAnnexHook preCommitAnnexHook
 		parent <- inRepo $ Git.Ref.sha branch
 		void $ inRepo $ Git.Branch.commit False commitmessage branch
 			(maybeToList parent)