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)

lua/diffs/fugitive.lua

@@ -27,19 +27,19 @@ function M.get_section_at_line(bufnr, lnum)
 end
 
 ---@param line string
----@return string?
+---@return string?, string?
 local function parse_file_line(line)
-  local renamed = line:match('^R[%s%d]*[^%s]+%s*->%s*(.+)$')
-  if renamed then
-    return vim.trim(renamed)
+  local old, new = line:match('^R%d*%s+(.-)%s+->%s+(.+)$')
+  if old and new then
+    return vim.trim(new), vim.trim(old)
   end
 
   local filename = line:match('^[MADRCU?][MADRCU%s]*%s+(.+)$')
   if filename then
-    return vim.trim(filename)
+    return vim.trim(filename), nil
   end
 
-  return nil
+  return nil, nil
 end
 
 ---@param line string
@@ -57,34 +57,34 @@ end
 
 ---@param bufnr integer
 ---@param lnum integer
----@return string?, diffs.FugitiveSection, boolean
+---@return string?, diffs.FugitiveSection, boolean, string?
 function M.get_file_at_line(bufnr, lnum)
   local lines = vim.api.nvim_buf_get_lines(bufnr, 0, -1, false)
   local current_line = lines[lnum]
 
   if not current_line then
-    return nil, nil, false
+    return nil, nil, false, nil
   end
 
   local section_header = parse_section_header(current_line)
   if section_header then
-    return nil, section_header, true
+    return nil, section_header, true, nil
   end
 
-  local filename = parse_file_line(current_line)
+  local filename, old_filename = parse_file_line(current_line)
   if filename then
     local section = M.get_section_at_line(bufnr, lnum)
-    return filename, section, false
+    return filename, section, false, old_filename
   end
 
   local prefix = current_line:sub(1, 1)
   if prefix == '+' or prefix == '-' or prefix == ' ' then
     for i = lnum - 1, 1, -1 do
       local prev_line = lines[i]
-      filename = parse_file_line(prev_line)
+      filename, old_filename = parse_file_line(prev_line)
       if filename then
         local section = M.get_section_at_line(bufnr, i)
-        return filename, section, false
+        return filename, section, false, old_filename
       end
       if prev_line:match('^%w+ %(') or prev_line == '' then
         break
@@ -92,7 +92,7 @@ function M.get_file_at_line(bufnr, lnum)
     end
   end
 
-  return nil, nil, false
+  return nil, nil, false, nil
 end
 
 ---@param bufnr integer
@@ -114,7 +114,7 @@ function M.diff_file_under_cursor(vertical)
   local bufnr = vim.api.nvim_get_current_buf()
   local lnum = vim.api.nvim_win_get_cursor(0)[1]
 
-  local filename, section, is_header = M.get_file_at_line(bufnr, lnum)
+  local filename, section, is_header, old_filename = M.get_file_at_line(bufnr, lnum)
 
   local repo_root = get_repo_root_from_fugitive(bufnr)
   if not repo_root then
@@ -141,13 +141,20 @@ function M.diff_file_under_cursor(vertical)
   end
 
   local filepath = repo_root .. '/' .. filename
+  local old_filepath = old_filename and (repo_root .. '/' .. old_filename) or nil
 
-  dbg('diff_file_under_cursor: %s (section: %s)', filename, section or 'unknown')
+  dbg(
+    'diff_file_under_cursor: %s (section: %s, old: %s)',
+    filename,
+    section or 'unknown',
+    old_filename or 'none'
+  )
 
   commands.gdiff_file(filepath, {
     vertical = vertical,
     staged = section == 'staged',
     untracked = section == 'untracked',
+    old_filepath = old_filepath,
   })
 end