changelog: parse entries with a shared limit-based helper

Replace parse_previous_version/parse_previous_version_from_str with
parse_changelog_entries(path, limit: Option<usize>), parsing up to the
given number of entries (None: the whole file) newest-first through the
same strict entry parser instead of a header-only scan. The single-entry
helpers stay as thin wrappers, and callers needing the previous entry
now get its full source name and version, not just the raw string.

src/build/mod.rs

@@ -255,7 +255,12 @@ pub fn run_source_build(
     // ------------------------------------------------------------------
     // 2. Metadata resolution
     // ------------------------------------------------------------------
-    let entry = crate::debian::parse_changelog_entry(&changelog_path)?;
+    // The current entry plus the one below it: the previous entry drives
+    // both the binNMU metadata references and the orig-tarball inclusion
+    // decision.
+    let mut entries = crate::debian::changelog::parse_changelog_entries(&changelog_path, Some(2))?;
+    let entry = entries.remove(0);
+    let previous_entry = entries.into_iter().next();
     let ctrl = ControlInfo::parse(&control_path)?;
 
     if let Some(u) = &ui {
@@ -265,7 +270,7 @@ pub fn run_source_build(
     // binNMU builds reference the *previous* (source) version in their
     // artifact metadata, like dpkg-genchanges/genbuildinfo do.
     let previous_version = if entry.binary_only {
-        crate::debian::changelog::parse_previous_version(&changelog_path)?
+        previous_entry.as_ref().map(|e| e.version.full())
     } else {
         None
     };