This script stops the build if git repository has uncommitted changes, useful when archiving because it makes sure releases (Archives) are created from a clean tree. Put this script in a target (say ArchiveHousekeeper) and update target scheme's Build section to add ArchiveHousekeeper as dependency for Archive operation.

ArchiveHousekeeper.sh

#!/bin/sh
if [ -z "$(git status --porcelain)" ]; then
    echo "Working directory clean, proceding to Archive"
    exit 0
else
    echo "error: Working directory dirty, stopping build"
    exit 1
fi