chlog: add -f/--file to target a specific changelog

The changelog path was hardcoded to debian/changelog in the series
detection, the entry generation and the editor invocation. Packages
keeping the changelog elsewhere (e.g. a staging copy) could not use
chlog on it.

Add -f/--file defaulting to debian/changelog, threaded through all
three uses. The library side was already parameterized on the file,
so no changelog.rs behavior changes beyond a test pinning the
non-default path.

src/main.rs

@@ -329,6 +329,7 @@ fn main() {
                     .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!(-f --file <path> "Path to the changelog file to generate into and edit (default: debian/changelog)").required(false).default_value("debian/changelog"))
                 .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")),
         )
@@ -667,6 +668,10 @@ fn main() {
                 .get_one::<usize>("limit")
                 .copied()
                 .unwrap_or(500);
+            let changelog_file = sub_matches
+                .get_one::<String>("file")
+                .map(|s| s.to_string())
+                .unwrap_or_else(|| "debian/changelog".to_string());
             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());
@@ -692,7 +697,7 @@ fn main() {
             let target_series = if let Some(s) = cli_series {
                 Some(s.to_string())
             } else {
-                let changelog_path = cwd.join("debian/changelog");
+                let changelog_path = cwd.join(&changelog_file);
                 match rt.block_on(pkh::changelog::series_candidates(&changelog_path)) {
                     Some(pkh::changelog::SeriesCandidates::Choose {
                         options,
@@ -720,7 +725,7 @@ fn main() {
             };
 
             let entry = match rt.block_on(pkh::changelog::generate_entry(
-                "debian/changelog",
+                &changelog_file,
                 Some(&cwd),
                 version,
                 target_series.as_deref(),
@@ -752,7 +757,7 @@ fn main() {
             };
             let _status = std::process::Command::new(&editor)
                 .current_dir(&cwd)
-                .args(["debian/changelog"])
+                .arg(&changelog_file)
                 .status()
                 .map_err(|e| {
                     error!(