Rename `diff` to `gitaly_diff` for clarity

Close https://gitlab.com/gitlab-org/gitlab/-/issues/219054

It can be confusing to have a method attribute `diff` and `@diff`
variable in the same scope.

lib/gitlab/git/diff.rb

@@ -224,18 +224,18 @@ module Gitlab
         end
       end
 
-      def init_from_gitaly(diff)
-        @diff = diff.respond_to?(:patch) ? encode!(diff.patch) : ''
-        @new_path = encode!(diff.to_path.dup)
-        @old_path = encode!(diff.from_path.dup)
-        @a_mode = diff.old_mode.to_s(8)
-        @b_mode = diff.new_mode.to_s(8)
-        @new_file = diff.from_id == BLANK_SHA
-        @renamed_file = diff.from_path != diff.to_path
-        @deleted_file = diff.to_id == BLANK_SHA
-        @too_large = diff.too_large if diff.respond_to?(:too_large)
-
-        collapse! if diff.respond_to?(:collapsed) && diff.collapsed
+      def init_from_gitaly(gitaly_diff)
+        @diff = gitaly_diff.respond_to?(:patch) ? encode!(gitaly_diff.patch) : ''
+        @new_path = encode!(gitaly_diff.to_path.dup)
+        @old_path = encode!(gitaly_diff.from_path.dup)
+        @a_mode = gitaly_diff.old_mode.to_s(8)
+        @b_mode = gitaly_diff.new_mode.to_s(8)
+        @new_file = gitaly_diff.from_id == BLANK_SHA
+        @renamed_file = gitaly_diff.from_path != gitaly_diff.to_path
+        @deleted_file = gitaly_diff.to_id == BLANK_SHA
+        @too_large = gitaly_diff.too_large if gitaly_diff.respond_to?(:too_large)
+
+        collapse! if gitaly_diff.respond_to?(:collapsed) && gitaly_diff.collapsed
       end
 
       def prune_diff_if_eligible