CI: Add tool to enforce the string freeze

- check for conventional translation tag
 - diff the generated translation files to enforce consistency

ci/txcheck.sh

@@ -0,0 +1,48 @@
+#! /bin/sh
+
+# Does the translation tag (from a previous txpush) exist?
+# This assumes that the release host has also locally done
+# a translations push, which works for the current development
+# workflow .. but it could be improved by looking for one of
+# the typical txpush log messages instead of the tag.
+if git describe translation > /dev/null 2>&1 ; then
+	:
+else
+	echo "! No 'translation' tag exists for enforcing the string-freeze."
+	exit 1
+fi
+# The tag exists, so now check that there's no unsaved changes
+if test `git describe` = `git describe --dirty` ; then
+	:
+else
+	echo "! There are local changes."
+	exit 1
+fi
+# No unsaved changes, collect names of relevant files
+test -f ".tx/config" || { echo "! No Transifex configuration is present." ; exit 1 ; }
+# Print part after = for each source_file line and delete all the rest
+TX_FILE_LIST=$( sed -e '/^source_file/s+.*=++p' -e d .tx/config )
+for f in $TX_FILE_LIST ; do
+	test -f $f || { echo "! Translation file '$f' does not exist." ; exit 1 ; }
+done
+
+# The state of translations here
+HEAD_SUM=$( cat $TX_FILE_LIST | sha256sum )
+
+# Check from the translation tag as well
+git worktree add build-txcheck translation
+( cd build-txcheck && sh ci/txpush.sh --no-tx ) || { echo "! Could not re-create translations." ; exit 1 ; }
+PREV_SUM=$( cd build-txcheck && cat $TX_FILE_LIST | sha256sum )
+
+if test "$HEAD_SUM" = "$PREV_SUM" ; then
+	:
+else
+	echo "! Translations have changed."
+	for f in $TX_FILE_LIST ; do
+		echo "! $f"
+		diff -u build-txcheck/$f $f
+	done
+	exit 1
+fi
+
+exit 0