Tweak id pretty print

Only show it if `details` flag is set, and display it like any other field.

nbdime/prettyprint.py

@@ -647,15 +647,19 @@ def c():
             # Write cell type and optionally number:
             numstr = "" if i is None else " %d" % i
             k = "%s cell%s" % (cell.get("cell_type"), numstr)
-            if "id" in cell:
-                k += f" [{cell['id']}]"
             pretty_print_key(k, prefix, config)
             c.called = True
     c.called = False
 
     if force_header:
         c()
 
+    id = cell.get("id")
+    if id and config.details:
+        # Write execution count if there (only source cells)
+        c()
+        pretty_print_item("id", id, key_prefix, config)
+
     execution_count = cell.get("execution_count")
     if execution_count and config.details:
         # Write execution count if there (only source cells)

nbdime/tests/filters/add_helper.py

@@ -34,7 +34,8 @@ def _build_arg_parser():
 def clean(notebook):
     if get_marker_cell(notebook) is None:
         notebook['cells'].append(nbformat.v4.new_raw_cell(
-            source=MARKER
+            source=MARKER,
+            id='filtered-cell',
         ))
 
 def smudge(notebook):

nbdime/tests/test_git_diffdriver.py

@@ -173,6 +173,7 @@
 
 ## inserted before /cells/6:
 +  raw cell:
++    id: filtered-cell
 +    source:
 +      nbdime test filter marker
 

nbdime/tests/test_prettyprint.py

@@ -200,6 +200,7 @@ def test_pretty_print_markdown_cell():
 def test_pretty_print_code_cell():
     cell = v4.new_code_cell(source='def foo():\n    return 4',
         execution_count=3,
+        id='code-cell-id',
         outputs=[
             v4.new_output('stream', name='stdout', text='some\ntext'),
             v4.new_output('display_data', {'text/plain': 'hello display'}),
@@ -213,6 +214,7 @@ def test_pretty_print_code_cell():
 
     assert lines == [
         '+code cell:',
+        '+  id: code-cell-id',
         '+  execution_count: 3',
         '+  source:',
         '+    def foo():',