git pre-commit hook for stopping you from commiting xibs that are in the new Xcode5 format that won't load in Xcode4. Surely there's a better way to find xibs that are in the new format than this, but it seems to work so far. To use: Save as .git/hooks/pre-commit.

pre-commit-xcode5xib-check

#!/bin/sh
#
# Pre-Commit Hook that checks for Xcode5 format XIB files as they won't be
# openable in Xcode4


# Redirect output to stderr.
exec 1>&2

xcode5xibs=`find . -name *.xib -exec grep "<document.*type=\"com.apple.InterfaceBuilder3.Cocoa.XIB\"" {} + | sed -e 's/:.*//'`

if [ "$xcode5xibs" != "" ]
then
	echo "Error: Attempt to add an Xcode5 xib file"
	echo "$xcode5xibs"
	exit 1
fi