cache: Mark buffer as modified on push

When a (task)wiki file is opened, viewports are refreshed but the buffer
is not marked as modified and therefore a subsequent :xa doesn't update
the file. This is, IMO, undesired.

vim documentation for the 'modified' option:

    This option is not set when a change is made to the buffer as the
    result of a BufNewFile, BufRead/BufReadPost, BufWritePost,
    FileAppendPost or VimLeave autocommand event.  See |gzip-example| for
    an explanation.

taskwiki/cache.py

@@ -22,10 +22,12 @@ class BufferProxy(object):
 
     def push(self):
         with util.current_line_preserved():
+            buffer = util.get_buffer(self.buffer_number)
             # Only set the buffer contents if the data is changed.
             # Avoids extra undo events with empty diff.
-            if util.get_buffer(self.buffer_number)[:] != self.data:
-                util.get_buffer(self.buffer_number)[:] = self.data
+            if buffer[:] != self.data:
+                buffer[:] = self.data
+                buffer.options['modified'] = True
 
     def __getitem__(self, index):
         try:

tests/test_viewport.py

@@ -1,4 +1,5 @@
 # -*- coding: utf-8 -*-
+import os
 from datetime import datetime
 from tests.base import MultiSyntaxIntegrationTest
 
@@ -614,3 +615,31 @@ class TestViewportDefaultPreservesTags(MultiSyntaxIntegrationTest):
         assert task['description'] == 'hard task'
         assert task['status'] == 'pending'
         assert task['tags'] == set(['work', 'hard'])
+
+
+class TestViewportBufferModified(MultiSyntaxIntegrationTest):
+
+    viminput = """
+    HEADER2(Work tasks |)
+    * [ ] a task  #{uuid}
+    """
+
+    tasks = [
+        dict(description="a task", tags=['work']),
+    ]
+
+    def execute(self):
+        testfile2 = os.path.join(self.dir, "testwiki2.txt")
+        testfile3 = os.path.join(self.dir, "testwiki3.txt")
+
+        self.command("w", regex="written$", lines=1)
+        self.command("w {}".format(testfile2), regex="written$", lines=1)
+        self.command("1w {}".format(testfile3), regex="written$", lines=1)
+
+        # testfile2 has the task, so refresh on open shouldn't change anything
+        self.client.edit(testfile2)
+        assert self.client.eval('&modified') == '0'
+
+        # testfile3 only has header, so refresh on open makes changes
+        self.client.edit(testfile3)
+        assert self.client.eval('&modified') == '1'