コミットメッセージの変更

修正したいメッセージのcommitが4件前の場合

git rebase -i HEAD~4

起動したエディタで対象のpickをrに変更

# Commands:
#  p, pick = use commit
#  r, reword = use commit, but edit the commit message
#  e, edit = use commit, but stop for amending
#  s, squash = use commit, but meld into previous commit
#  f, fixup = like "squash", but discard this commit's log message
#  x, exec = run command (the rest of the line) using shell
#
# If you remove a line here THAT COMMIT WILL BE LOST.
# However, if you remove everything, the rebase will be aborted.

続いて起動したエディタ(例vi Readonly属性)ファイルのメッセージを強制書き換え(w!)
参考
http://tobysoft.net/wiki/index.php?git%2F%A5%B3%A5%DF%A5%C3%A5%C8%A5%ED%A5%B0%A4%F2%BD%A4%C0%B5%A4%B9%A4%EB%CA%FD%CB%A1#s2ae23eb