feat: new file::Index::highest_offset() method (#279)

With it it's simpler to figure out from where to read trailing
checksums.

git-chunk/src/file/decode.rs

@@ -18,6 +18,9 @@ mod error {
             ChunkSizeOutOfBounds { offset: crate::file::Offset, file_length: u64 } {
                 display("The chunk offset {} went past the file of length {} - was it truncated?", offset, file_length)
             }
+            NonIncrementalChunkOffsets {
+                display("All chunk offsets must be incrementing.")
+            }
             DuplicateChunk(kind: crate::Kind) {
                 display("The chunk of kind {:#016x} was encountered more than once", kind)
             }
@@ -78,6 +81,9 @@ impl file::Index {
                     file_length: data_len,
                 });
             }
+            if next_offset <= offset {
+                return Err(Error::NonIncrementalChunkOffsets);
+            }
             chunks.push(index::Entry {
                 kind,
                 offset: Range {

git-chunk/src/file/index.rs

@@ -82,4 +82,10 @@ impl Index {
         let offset = self.offset_by_kind(kind, name)?;
         Ok(&data[crate::into_usize_range(offset).ok_or(data_by_kind::Error::FileTooLarge)?])
     }
+
+    /// Return the end offset lf the last chunk, which is the highest offset as well.
+    /// It's definitely available as we have one or more chunks.
+    pub fn highest_offset(&self) -> crate::file::Offset {
+        self.chunks.last().expect("at least one chunk").offset.end
+    }
 }