fix(fugitive): handle renamed files correctly

Parse both old and new filenames from rename lines (R old -> new).
When diffing staged renames, use old filename as base to correctly
show content changes rather than treating the file as entirely new.

Also adds comprehensive tests for filename edge cases:
- Double extensions, hyphens, underscores, dotfiles
- Deep nested paths, complex renames
- Documents known limitation with filenames containing ' -> '

lua/diffs/commands.lua

@@ -92,6 +92,7 @@ end
 ---@field vertical? boolean
 ---@field staged? boolean
 ---@field untracked? boolean
+---@field old_filepath? string
 
 ---@param filepath string
 ---@param opts? diffs.GdiffFileOpts
@@ -104,6 +105,8 @@ function M.gdiff_file(filepath, opts)
     return
   end
 
+  local old_rel_path = opts.old_filepath and git.get_relative_path(opts.old_filepath) or rel_path
+
   local old_lines, new_lines, err
   local diff_label
 
@@ -116,7 +119,7 @@ function M.gdiff_file(filepath, opts)
     end
     diff_label = 'untracked'
   elseif opts.staged then
-    old_lines, err = git.get_file_content('HEAD', filepath)
+    old_lines, err = git.get_file_content('HEAD', opts.old_filepath or filepath)
     if not old_lines then
       old_lines = {}
     end
@@ -126,9 +129,9 @@ function M.gdiff_file(filepath, opts)
     end
     diff_label = 'staged'
   else
-    old_lines, err = git.get_index_content(filepath)
+    old_lines, err = git.get_index_content(opts.old_filepath or filepath)
     if not old_lines then
-      old_lines, err = git.get_file_content('HEAD', filepath)
+      old_lines, err = git.get_file_content('HEAD', opts.old_filepath or filepath)
       if not old_lines then
         old_lines = {}
         diff_label = 'untracked'
@@ -144,7 +147,7 @@ function M.gdiff_file(filepath, opts)
     end
   end
 
-  local diff_lines = generate_unified_diff(old_lines, new_lines, rel_path, rel_path)
+  local diff_lines = generate_unified_diff(old_lines, new_lines, old_rel_path, rel_path)
 
   if #diff_lines == 0 then
     vim.notify('[diffs.nvim]: no changes', vim.log.levels.INFO)