Fix metadata hook script to support non-ascii characters

tips/automatically_adding_metadata/pre-commit-annex: Fix to not silently
skip filenames containing non-ascii characters.

git diff-index defaults to munging non-ascii characters. Using -z makes
it not do that, and then we just change the nulls to newlines.

This commit was sponsored by Jochen Bartl on Patreon.

doc/tips/automatically_adding_metadata/pre-commit-annex

@@ -112,7 +112,7 @@ if [ -n "$*" ]; then
 		process "$f"
 	done
 else
-	git diff-index --name-only --cached $against | while read f; do
+	git diff-index -z --name-only --cached $against | sed 's/\x00/\n/g' | while read f; do
 		process "$f"
 	done
 fi