feat(fugitive): line position tracking for keymaps

When pressing `du`/`dU` from a hunk line in the fugitive status buffer
(after expanding with `=`), the unified diff now opens at the
corresponding line instead of line 1.

Implementation:
- `fugitive.get_hunk_position()` returns @@ header and offset when on a hunk line
- `commands.find_hunk_line()` finds matching @@ header in diff buffer
- `commands.gdiff_file()` accepts optional `hunk_position` and jumps after opening

Also updates @phanen's README credit for the previous two fixes.

Closes #65

lua/diffs/commands.lua

@@ -3,6 +3,18 @@ local M = {}
 local git = require('diffs.git')
 local dbg = require('diffs.log').dbg
 
+---@param diff_lines string[]
+---@param hunk_position { hunk_header: string, offset: integer }
+---@return integer?
+function M.find_hunk_line(diff_lines, hunk_position)
+  for i, line in ipairs(diff_lines) do
+    if line == hunk_position.hunk_header then
+      return i + hunk_position.offset
+    end
+  end
+  return nil
+end
+
 ---@param old_lines string[]
 ---@param new_lines string[]
 ---@param old_name string
@@ -98,6 +110,7 @@ end
 ---@field staged? boolean
 ---@field untracked? boolean
 ---@field old_filepath? string
+---@field hunk_position? { hunk_header: string, offset: integer }
 
 ---@param filepath string
 ---@param opts? diffs.GdiffFileOpts
@@ -175,6 +188,14 @@ function M.gdiff_file(filepath, opts)
   vim.cmd(opts.vertical and 'vsplit' or 'split')
   vim.api.nvim_win_set_buf(0, diff_buf)
 
+  if opts.hunk_position then
+    local target_line = M.find_hunk_line(diff_lines, opts.hunk_position)
+    if target_line then
+      vim.api.nvim_win_set_cursor(0, { target_line, 0 })
+      dbg('jumped to line %d for hunk', target_line)
+    end
+  end
+
   dbg('opened diff buffer %d for %s (%s)', diff_buf, rel_path, diff_label)
 
   vim.schedule(function()

lua/diffs/fugitive.lua

@@ -95,6 +95,42 @@ function M.get_file_at_line(bufnr, lnum)
   return nil, nil, false, nil
 end
 
+---@class diffs.HunkPosition
+---@field hunk_header string
+---@field offset integer
+
+---@param bufnr integer
+---@param lnum integer
+---@return diffs.HunkPosition?
+function M.get_hunk_position(bufnr, lnum)
+  local lines = vim.api.nvim_buf_get_lines(bufnr, 0, lnum, false)
+  local current = lines[lnum]
+
+  if not current then
+    return nil
+  end
+
+  local prefix = current:sub(1, 1)
+  if prefix ~= '+' and prefix ~= '-' and prefix ~= ' ' then
+    return nil
+  end
+
+  for i = lnum - 1, 1, -1 do
+    local line = lines[i]
+    if line:match('^@@.-@@') then
+      return {
+        hunk_header = line,
+        offset = lnum - i,
+      }
+    end
+    if line:match('^[MADRCU?!]%s') or line:match('^%w+ %(') then
+      break
+    end
+  end
+
+  return nil
+end
+
 ---@param bufnr integer
 ---@return string?
 local function get_repo_root_from_fugitive(bufnr)
@@ -142,12 +178,14 @@ function M.diff_file_under_cursor(vertical)
 
   local filepath = repo_root .. '/' .. filename
   local old_filepath = old_filename and (repo_root .. '/' .. old_filename) or nil
+  local hunk_position = M.get_hunk_position(bufnr, lnum)
 
   dbg(
-    'diff_file_under_cursor: %s (section: %s, old: %s)',
+    'diff_file_under_cursor: %s (section: %s, old: %s, hunk_offset: %s)',
     filename,
     section or 'unknown',
-    old_filename or 'none'
+    old_filename or 'none',
+    hunk_position and tostring(hunk_position.offset) or 'none'
   )
 
   commands.gdiff_file(filepath, {
@@ -155,6 +193,7 @@ function M.diff_file_under_cursor(vertical)
     staged = section == 'staged',
     untracked = section == 'untracked',
     old_filepath = old_filepath,
+    hunk_position = hunk_position,
   })
 end