Clearly not with every git:

  $ git commit --amend a98fd2e95dde78041132b6281f6c2127712507f5
  error: pathspec 'a98fd2e95dde78041132b6281f6c2127712507f5' did not match any file(s) known to git.
  $ git --version
  git version 1.7.11.msysgit.1