merge_tools: rename MergeTool::Internal -> MergeTool::Builtin

The name we will expose to the user is `builtin`, so this changes the internal messaging to match that.

cli/src/merge_tools/mod.rs

@@ -108,7 +108,7 @@ pub fn run_mergetool(
 
     let editor = get_merge_tool_from_settings(ui, settings)?;
     match editor {
-        MergeTool::Internal => unimplemented!("run_mergetool with internal mergetool"),
+        MergeTool::Builtin => unimplemented!("run_mergetool with internal mergetool"),
         MergeTool::External(editor) => external::run_mergetool_external(
             &editor, file_merge, content, repo_path, conflict, tree,
         ),
@@ -126,7 +126,7 @@ pub fn edit_diff(
     // Start a diff editor on the two directories.
     let editor = get_diff_editor_from_settings(ui, settings)?;
     match editor {
-        MergeTool::Internal => unimplemented!("run_mergetool with internal mergetool"),
+        MergeTool::Builtin => unimplemented!("run_mergetool with internal mergetool"),
         MergeTool::External(editor) => edit_diff_external(
             editor,
             left_tree,
@@ -140,7 +140,7 @@ pub fn edit_diff(
 
 #[derive(Clone, Debug, Eq, PartialEq)]
 pub enum MergeTool {
-    Internal,
+    Builtin,
     External(ExternalMergeTool),
 }