Keep "#" lines in a commit message where the host strips comments [patch]

CLAUDE.md

@@ -366,6 +366,12 @@ Non-obvious, load-bearing design points:
     `Commit()` reuses the `Log()` vector for its read-back, so without the flag a signed commit that
     git did make is reported to the caller as a failure, and a retry makes a duplicate.
 
+16. **`Commit()` passes `--cleanup=whitespace`.** With `commit.cleanup=strip` (or `scissors`) in the
+    host's config, git treats every `#`-leading line of a `--message` as a comment and drops it, so
+    `#123 fixed the crash` silently vanishes from the body, and a `#42: fix login` subject aborts the
+    commit as empty. `whitespace` is git's own default for a message that never opens an editor, so
+    the stored message on an unconfigured host is unchanged.
+
 **Hosting layer.** `GitProvider` is an abstract base with two implementations: `GitHubProvider` over
 Octokit, and `AzureDevOpsProvider` over a raw `HttpClient` — Azure DevOps has no client library this
 library uses (see the dependency note below). Both go through the same shape: every request-issuing

GitIntegration.Test/Builders/GitCommitBuilderTests.cs

@@ -39,6 +39,7 @@
 			"-c", "core.quotepath=false",
 			"-c", "color.ui=false",
 			"commit",
+			"--cleanup=whitespace",
 			"--message", "subject here",
 		];
 		CollectionAssert.AreEqual(expectedArguments, builder.BuildArguments().ToArray());
@@ -61,6 +62,22 @@
 		Assert.AreEqual("body text", arguments[subject + 2]);
 	}
 
+	[TestMethod]
+	public void PinsTheMessageCleanupModeAheadOfTheMessage()
+	{
+		// commit.cleanup=strip on the host would otherwise drop every "#"-leading line of the message.
+		RecordingGitProcessRunner runner = new();
+		GitCommitBuilder builder = new(runner, TestPaths.Root, Message);
+
+		_ = builder.WithBody("#123 fixed the crash").AllowEmpty().StageTrackedFiles();
+
+		string[] arguments = [.. builder.BuildArguments()];
+		Assert.AreEqual(1, Array.FindAll(arguments, a => a.StartsWith("--cleanup", StringComparison.Ordinal)).Length);
+		Assert.IsLessThan(
+			Array.IndexOf(arguments, "--message"),
+			Array.IndexOf(arguments, "--cleanup=whitespace"));
+	}
+
 	[TestMethod]
 	public void MapsTheOptionFlags()
 	{

GitIntegration.Test/Integration/GitRoundTripTests.cs

@@ -361,6 +361,40 @@
 		Assert.IsTrue(suppressed.IsClean);
 	}
 
+	[TestMethod]
+	public async Task CommitKeepsHashLinesEvenWhereTheHostStripsCommentsAsync()
+	{
+		// commit.cleanup is a host setting, and under "strip" git treats every "#"-leading line as a
+		// comment. Written into the throwaway repository's own config for the same reason as the
+		// status.showUntrackedFiles test above: same variable, nearest scope, host left alone.
+		CancellationToken cancellationToken = TestContext.CancellationTokenSource.Token;
+		await IntegrationGitFixture.RequireGitAsync(cancellationToken).ConfigureAwait(false);
+
+		using TemporaryRepository temporary = new();
+		GitRepository repository = await InitializeAsync(temporary, cancellationToken).ConfigureAwait(false);
+
+		_ = await new GitTextBuilder(
+			repository.ProcessRunner!, repository.LocalPath, "config", "commit.cleanup", "strip")
+			.ExecuteAsync(cancellationToken).ConfigureAwait(false);
+
+		temporary.WriteFile("a.txt", "one\n");
+		_ = await repository.Add().All().ExecuteAsync(cancellationToken).ConfigureAwait(false);
+		GitCommit commit = await repository.Commit("Subject".As<GitCommitMessage>())
+			.WithBody("#123 fixed the crash\nsecond")
+			.ExecuteAsync(cancellationToken).ConfigureAwait(false);
+
+		Assert.AreEqual("#123 fixed the crash\nsecond", commit.Body, "Commit() dropped the body's \"#\" line.");
+
+		// A "#"-leading subject is the whole message as far as strip is concerned; without the pin
+		// git aborts with "empty commit message" instead of recording it.
+		temporary.WriteFile("a.txt", "two\n");
+		_ = await repository.Add().All().ExecuteAsync(cancellationToken).ConfigureAwait(false);
+		GitCommit hashSubject = await repository.Commit("#42: fix login".As<GitCommitMessage>())
+			.ExecuteAsync(cancellationToken).ConfigureAwait(false);
+
+		Assert.AreEqual("#42: fix login", hashSubject.Subject);
+	}
+
 	[TestMethod]
 	public async Task BranchCreateCheckoutAndDeleteRoundTripAsync()
 	{

GitIntegration/Builders/GitCommitBuilder.cs

@@ -129,6 +129,12 @@ protected override void AppendVerbArguments(ICollection<string> arguments)
 			arguments.Add("--author=" + _author);
 		}
 
+		// Pinned because commit.cleanup is a host setting: under "strip" or "scissors" git treats every
+		// "#"-leading line as a comment and drops it, so "#123 fixed the crash" vanishes from the body
+		// and a "#42: fix login" subject aborts the commit as empty. "whitespace" is git's own default
+		// for a message that never opens an editor, so an unconfigured host stores the same message.
+		arguments.Add("--cleanup=whitespace");
+
 		// --message rather than -m, and repeated for the body: git joins repeated values with a
 		// blank line between them, which is precisely the subject-then-body convention.
 		arguments.Add("--message");