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.

debian/changelog

@@ -16,6 +16,9 @@ git-annex (5.20140228) UNRELEASED; urgency=medium
     using !tag and field!=value.
   * vadd: Allow listing multiple desired values for a field.
   * view: Refuse to enter a view when no branch is currently checked out.
+  * metadata: To only set a field when it's not already got a value, use
+    -s field?=value
+  * Run .git/hooks/pre-commit-annex whenever a commit is made.
 
  -- Joey Hess <joeyh@debian.org>  Fri, 28 Feb 2014 14:52:15 -0400