chlog: cap the no-tag history search with --limit

Without a version tag, the boundary of a new entry was found by walking
the whole history and diffing each commit's tree against the changelog
path. On large repositories that per-commit tree diff takes 10+ minutes.

Add --limit (default 500, 0 = unlimited) capping that walk: when no
changelog commit is found within the last N commits, the oldest visited
commit becomes the boundary — the version tag probe is skipped there,
since it could jump the boundary arbitrarily far back and defeat the
cap — and a warning explains the fallback. A found version tag still
takes precedence and is unaffected by the limit.

src/main.rs

@@ -328,7 +328,9 @@ fn main() {
                 .arg(arg!(--nmu "Number the entry as a non-maintainer upload (1.0-1 becomes 1.0-1.1, native 1.0 becomes 1.0+nmu1)").required(false)
                     .conflicts_with("rebuild"))
                 .arg(arg!(--rebuild "Number the entry as a no-change rebuild (1.0-1 becomes 1.0-1build1)").required(false))
-                .arg(arg!(-v --version <version> "Target version (overrides the --backport/--nmu/--rebuild numbering)").required(false)),
+                .arg(arg!(-v --version <version> "Target version (overrides the --backport/--nmu/--rebuild numbering)").required(false))
+                .arg(arg!(--limit <n> "Cap how far back the history is searched for the previous changelog entry when no version tag exists (0 = unlimited)").required(false)
+                    .value_parser(clap::value_parser!(usize)).default_value("500")),
         )
         .subcommand(
             Command::new("build")
@@ -661,6 +663,10 @@ fn main() {
             info!("Done.");
         }
         Some(("chlog", sub_matches)) => {
+            let history_limit = sub_matches
+                .get_one::<usize>("limit")
+                .copied()
+                .unwrap_or(500);
             let cwd = current_dir_or_exit();
             let version = sub_matches.get_one::<String>("version").map(|s| s.as_str());
             let cli_series = sub_matches.get_one::<String>("series").map(|s| s.as_str());
@@ -719,6 +725,7 @@ fn main() {
                 version,
                 target_series.as_deref(),
                 kind,
+                history_limit,
             )) {
                 Ok(entry) => entry,
                 Err(e) => {