feat(git): add index and working tree content retrieval

Adds functions for accessing git index content and working tree files:
- get_index_content() retrieves file from staging area via :0:path
- get_working_content() reads file directly from disk
- file_exists_in_index() checks if file is staged
- file_exists_at_revision() checks if file exists at given revision

lua/diffs/git.lua

@@ -45,4 +45,69 @@ function M.get_relative_path(filepath)
   return vim.fn.fnamemodify(filepath, ':.')
 end
 
+---@param filepath string
+---@return string[]?, string?
+function M.get_index_content(filepath)
+  local repo_root = M.get_repo_root(filepath)
+  if not repo_root then
+    return nil, 'not in a git repository'
+  end
+
+  local rel_path = M.get_relative_path(filepath)
+  if not rel_path then
+    return nil, 'could not determine relative path'
+  end
+
+  local result = vim.fn.systemlist({ 'git', '-C', repo_root, 'show', ':0:' .. rel_path })
+  if vim.v.shell_error ~= 0 then
+    return nil, 'file not in index'
+  end
+  return result, nil
+end
+
+---@param filepath string
+---@return string[]?, string?
+function M.get_working_content(filepath)
+  if vim.fn.filereadable(filepath) ~= 1 then
+    return nil, 'file not readable'
+  end
+  local lines = vim.fn.readfile(filepath)
+  return lines, nil
+end
+
+---@param filepath string
+---@return boolean
+function M.file_exists_in_index(filepath)
+  local repo_root = M.get_repo_root(filepath)
+  if not repo_root then
+    return false
+  end
+
+  local rel_path = M.get_relative_path(filepath)
+  if not rel_path then
+    return false
+  end
+
+  vim.fn.system({ 'git', '-C', repo_root, 'ls-files', '--stage', '--', rel_path })
+  return vim.v.shell_error == 0
+end
+
+---@param revision string
+---@param filepath string
+---@return boolean
+function M.file_exists_at_revision(revision, filepath)
+  local repo_root = M.get_repo_root(filepath)
+  if not repo_root then
+    return false
+  end
+
+  local rel_path = M.get_relative_path(filepath)
+  if not rel_path then
+    return false
+  end
+
+  vim.fn.system({ 'git', '-C', repo_root, 'cat-file', '-e', revision .. ':' .. rel_path })
+  return vim.v.shell_error == 0
+end
+
 return M