Implement merging of PO file comments

Issue #922
Signed-off-by: Michal Čihař <michal@cihar.com>

weblate/trans/formats.py

@@ -920,6 +920,14 @@ class PoFormat(FileFormat):
 
         self.store.updateheader(**update)
 
+        header = self.store.header()
+        newheader = otherstore.store.header()
+        if not header or not newheader:
+            return
+
+        header.removenotes()
+        header.addnote(newheader.getnotes())
+
 
 @register_fileformat
 class PoMonoFormat(PoFormat):

weblate/trans/tests/data/cs.po

@@ -2,6 +2,7 @@
 # Copyright (C) 2012 THE PACKAGE'S COPYRIGHT HOLDER
 # This file is distributed under the same license as the PACKAGE package.
 # Automatically generated, 2012.
+# Testing Weblate, 2015.
 #
 msgid ""
 msgstr ""

weblate/trans/tests/test_files.py

@@ -112,6 +112,10 @@ class ImportTest(ImportBaseTest):
         self.assertEqual(
             header['Language-Team'], 'Test Team <noreply@weblate.org>'
         )
+        self.assertIn(
+            'Testing Weblate, 2015.',
+            unit.translation.store.store.header().getnotes()
+        )
 
     def test_import_author(self):
         '''