prepare-release: Add merge-translations command

This command allows merging translations from another branch. This
is to be used strictly downwards, that is, translations should be
merged from newest to oldest branch, without skipping any active
branch, to ensure the best result.

For example, consider branches 1 and 2 have the strings "a" and "b"
to translate, but master only has the strings "a" and "c". Now, 2
has more translations for "b" than 1. By merging master into 2, and
2 into 1, 1 also gets any translations for "b" from 2.

prepare-release

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/bin/bash
 set -e
 
 cd "$(readlink -f $(dirname $0))"
@@ -341,6 +341,23 @@ elif [ "$1" = 'spellcheckers' -o "$1" = 'lint' ]; then
 				-e '^I: po/es.po: duplicate-header-field X-POFile-SpellExtra$' \
 			|| true
 	fi
+elif [ "$1" = "merge-translations" ]; then
+	if [ -z "$2" ]; then
+		echo "Usage:\t$0 $1 <branch to merge from>" >&2
+		exit 1
+	fi
+	for i in {doc/,}po/*.po ; do
+		# 1. concatenate the translations, picking new translations
+		# 2. merge the translations so we only have matching translations left
+		# 3. remove any newly introduced obsolete translations (only in $2)
+		# 4. concatenate again to restore "old" obsolete translations
+		# 5. write output
+		msgcat --use-first <(git show $2:$i) $i \
+			| msgmerge --no-fuzzy --previous - $i \
+			| msgattrib --no-obsolete -  \
+			| msgcat --use-first - $i \
+			| sponge $i
+	done
 else
 	echo >&1 "Usage:\t$0 pre-export
 \t$0 pre-build
@@ -373,6 +390,10 @@ runs all tests and generates a html report in the end.
 
 »spellcheckers« runs »codespell« and »spellintian« on the appropiate files and
 filters out obvious false positives.
+
+\t$0 merge-translations branch
+
+Merge translations from the given branch.
 "
 
 fi