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/debian/mod.rs

@@ -20,8 +20,8 @@ pub mod files;
 pub mod version;
 
 pub use changelog::{
-    ChangelogEntry, parse_changelog_entry, parse_changelog_entry_from_str,
-    parse_previous_version_from_str,
+    ChangelogEntry, parse_changelog_entries, parse_changelog_entries_from_str,
+    parse_changelog_entry, parse_changelog_entry_from_str,
 };
 pub use checksums::{ChecksumKind, Entry as ChecksumEntry, FileChecksums};
 pub use control::{