add platform tests and implementation

That way, the platform can be used to perform actual merges.
This will also be a good chance to try the API.

Author: Byron

gix-merge/src/blob/mod.rs

@@ -1,6 +1,7 @@
 // TODO: remove this - only needed while &mut Vec<u8> isn't used.
 #![allow(clippy::ptr_arg)]
 
+use crate::blob::platform::{DriverChoice, ResourceRef};
 use bstr::BString;
 use std::path::PathBuf;
 
@@ -157,3 +158,20 @@ pub struct Platform {
     /// The way we convert resources into mergeable states.
     filter_mode: pipeline::Mode,
 }
+
+/// The product of a [`prepare_merge()`](Platform::prepare_merge()) call to finally
+/// perform the merge and retrieve the merge results.
+#[derive(Copy, Clone)]
+pub struct PlatformRef<'parent> {
+    /// The platform that hosts the resources, used to access drivers.
+    pub(super) parent: &'parent Platform,
+    /// The current or our side of the merge operation.
+    pub current: ResourceRef<'parent>,
+    /// The ancestor or base of the merge operation.
+    pub ancestor: ResourceRef<'parent>,
+    /// The other or their side of the merge operation.
+    pub other: ResourceRef<'parent>,
+    /// Which driver to use according to the resource's configuration,
+    /// using the path of `current` to read git-attributes.
+    pub driver: DriverChoice,
+}