feat: add entry::Mode::is_submodule()

Author: Byron

gix-index/src/entry/mode.rs

@@ -1,11 +1,16 @@
 use crate::entry::Mode;
 
 impl Mode {
-    /// Return true if this is a sparse entry, as it points to a directory which usually isn't what an 'unsparse' index tracks.
+    /// Return `true` if this is a sparse entry, as it points to a directory which usually isn't what an 'unsparse' index tracks.
     pub fn is_sparse(&self) -> bool {
         *self == Self::DIR
     }
 
+    /// Return `true` if this is a submodule entry.
+    pub fn is_submodule(&self) -> bool {
+        *self == Self::DIR | Self::SYMLINK
+    }
+
     /// Compares this mode to the file system version ([`std::fs::symlink_metadata`])
     /// and returns the change needed to update this mode to match the file.
     ///