amend: respect git config commit.cleanup

Git commit has several configuration options
for how to strip comment lines out of the commit
text. We generally want to support these because
they affect whether we throw away prefixed lines
that could be important text to the user.

"strip" is just the default way we've been doing it, so
refactor to also support "scissors" which has a different
comment and removal method. "whitespace" and "verbatim"
are simpler strategies that we can support as well, but get
no additional comment.

Fixes: #148

revup/amend.py

@@ -16,8 +16,14 @@
 
 RE_TOPIC_WITH_MODIFIERS = re.compile(r"(?P<topic>[a-zA-Z\-_0-9]+)(?P<modifiers>[\^~]+[0-9]*)?")
 
+CLEANUP_SCISSOR_LINE = r"------------------------ >8 ------------------------"
+CLEANUP_SCISSOR_COMMENT = """Do not modify or remove the line above.
+Everything below it will be ignored."""
+CLEANUP_STRIP_COMMENT = """Please enter the commit message for your changes. Lines starting
+with '#' will be ignored, and an empty message aborts the amend."""
 
-def invoke_editor_for_commit_msg(
+
+async def invoke_editor_for_commit_msg(
     git_ctx: git.Git, editor: str, topic_summary: str, commit_msg: str, cache_stat: str, stat: str
 ) -> str:
     """
@@ -32,19 +38,38 @@ def invoke_editor_for_commit_msg(
         full_stat.append(f"Original commit:\n{stat}")
     stat_text = "\n\n".join(full_stat)
 
-    comment_text = f"""\nPlease enter the commit message for your changes. Lines starting
-with '#' will be ignored, and an empty message aborts the commit.\n{topic_summary}\n{stat_text}"""
-    comment_text = "\n# ".join(comment_text.splitlines())
+    # Respect the configured option for commit msg cleanup.
+    cleanup_ret, cleanup_type = await git_ctx.git("config", "commit.cleanup", raiseonerror=False)
+    if cleanup_ret != 0:
+        # git's default if the message is being edited (which if we've reached this it is)
+        cleanup_type = "strip"
+
+    comments = f"{topic_summary}\n{stat_text}"
+    if cleanup_type == "scissors":
+        comments = f"\n{CLEANUP_SCISSOR_LINE}\n{CLEANUP_SCISSOR_COMMENT}\n{comments}"
+    elif cleanup_type == "strip":
+        comments = f"\n{CLEANUP_STRIP_COMMENT}\n{comments}"
+
+    comments = "\n# ".join(comments.splitlines())
 
     with open(git_ctx.get_scratch_dir() + "/COMMIT_EDITMSG", mode="w") as temp_file:
-        temp_file.write(f"{commit_msg}\n{comment_text}")
+        temp_file.write(f"{commit_msg}\n{comments}")
 
     subprocess.check_call((*shlex.split(editor), temp_file.name))
     with open(temp_file.name, "r") as editor_file:
         msg = editor_file.read()
 
-    # Strip out comment lines
-    return re.sub(r"^\s*#.*$", "", msg, flags=re.M).strip()
+    if cleanup_type == "strip":
+        # Strip out comment lines
+        msg = re.sub(r"^#.*$\n", "", msg, flags=re.M)
+    elif cleanup_type == "scissors":
+        msg = msg.split("# " + CLEANUP_SCISSOR_LINE)[0]
+
+    if cleanup_type != "verbatim":
+        # git would also strip per-line trailing whitespace and collapse consecutive empty lines.
+        msg = msg.strip()
+
+    return msg
 
 
 async def get_topic_summary(topics: topic_stack.TopicStack) -> str:
@@ -165,7 +190,7 @@ async def get_has_unstaged() -> bool:
         stack[0].commit_msg = ""
 
     if args.edit and not args.drop:
-        new_msg = invoke_editor_for_commit_msg(
+        new_msg = await invoke_editor_for_commit_msg(
             git_ctx,
             git_ctx.editor,
             await get_topic_summary(topics) if args.parse_topics else "",