Support for 'sdir' extension (#293)

It's really only a marker and it doesn't seem to matter much
as not reading it won't matter for the entire git test suite.
Nor does it matter when it's not written, and I wonder if
a sparse checkout without a directory is even possible. In the latter
case one would have to remember sparseness differently as there is no
entry that could carry the 'sparse' file mode.

Author: Byron

git-bitmap/src/lib.rs

@@ -50,7 +50,6 @@ pub mod ewah {
         }
 
         let (rlw, data) = decode::u32(data).ok_or(decode::Error::Corrupt("eof while reading run length width"))?;
-        dbg!(rlw);
 
         Ok((
             Vec {

git-index/src/decode/mod.rs

@@ -198,9 +198,14 @@ impl State {
         let EntriesOutcome {
             entries,
             path_backing,
-            is_sparse,
+            mut is_sparse,
         } = entries;
-        let extension::decode::Outcome { tree, link } = ext;
+        let extension::decode::Outcome {
+            tree,
+            link,
+            is_sparse: is_sparse_from_ext, // a marker is needed in case there are no directories
+        } = ext;
+        is_sparse |= is_sparse_from_ext;
 
         Ok((
             State {

git-index/src/extension/decode.rs

@@ -41,6 +41,13 @@ pub fn all(maybe_beginning_of_extensions: &[u8], object_hash: git_hash::Kind) ->
             extension::index_entry_offset_table::SIGNATURE => {} // not relevant/obtained already
             mandatory if mandatory[0].is_ascii_lowercase() => match mandatory {
                 extension::link::SIGNATURE => ext.link = extension::link::decode(ext_data, object_hash)?.into(),
+                extension::sparse::SIGNATURE => {
+                    if !ext_data.is_empty() {
+                        // only used as a marker, if this changes we need this implementation.
+                        return Err(Error::MandatoryUnimplemented(mandatory));
+                    }
+                    ext.is_sparse = true
+                }
                 unknown => return Err(Error::MandatoryUnimplemented(unknown)),
             },
             _unknown => {} // skip unknown extensions, too
@@ -53,4 +60,5 @@ pub fn all(maybe_beginning_of_extensions: &[u8], object_hash: git_hash::Kind) ->
 pub struct Outcome {
     pub tree: Option<extension::Tree>,
     pub link: Option<extension::Link>,
+    pub is_sparse: bool,
 }

git-index/src/extension/mod.rs

@@ -35,6 +35,13 @@ pub(crate) mod end_of_index_entry;
 
 pub(crate) mod index_entry_offset_table;
 
+pub mod sparse {
+    use crate::extension::Signature;
+
+    /// Only used as an indicator
+    pub const SIGNATURE: Signature = *b"sdir";
+}
+
 pub mod link {
     use crate::extension::{Link, Signature};
     use crate::util::split_at_pos;