feat(changelog): Improve whitespace in changelog

Ensure there is at least one blank line separating old and new content.
We already separate the log entries for each release with blank lines,
so separate pre-existing content from the first release with a blank
line as well. If the pre-existing content ends in a blank line, then
don't add an extra one.

Author: Kurt-von-Laven

commitizen/changelog.py

@@ -275,6 +275,9 @@ def incremental_build(new_content: str, lines: List[str], metadata: Dict) -> Lis
 
         output_lines.append(line)
     if not isinstance(latest_version_position, int):
+        if output_lines and output_lines[-1].strip():
+            # Ensure at least one blank line between existing and new content.
+            output_lines.append("\n")
         output_lines.append(new_content)
     return output_lines
 

tests/commands/test_changelog_command.py

@@ -320,6 +320,30 @@ def test_changelog_multiple_incremental_do_not_add_new_lines(
     assert out.startswith("#")
 
 
+@pytest.mark.usefixtures("tmp_commitizen_project")
+def test_changelog_incremental_newline_separates_new_content_from_old(
+    mocker, changelog_path
+):
+    """Test for https://github.com/commitizen-tools/commitizen/issues/509"""
+    with open(changelog_path, "w") as f:
+        f.write("Pre-existing content that should be kept\n")
+
+    create_file_and_commit("feat: add more cat videos")
+
+    testargs = ["cz", "changelog", "--incremental"]
+
+    mocker.patch.object(sys, "argv", testargs)
+    cli.main()
+
+    with open(changelog_path, "r") as f:
+        out = f.read()
+
+    assert (
+        out
+        == "Pre-existing content that should be kept\n\n## Unreleased\n\n### Feat\n\n- add more cat videos\n"
+    )
+
+
 def test_changelog_without_revision(mocker, tmp_commitizen_project):
     changelog_file = tmp_commitizen_project.join("CHANGELOG.md")
     changelog_file.write(