merge: cleanup messages like commit

merge: cleanup messages like commit

This change allows git-merge messages to be cleaned up with the
commit.cleanup configuration or --cleanup option, just like how
git-commit does it.

We also give git-pull the option of --cleanup so that it can also take
advantage of this change.

Finally, add testing to ensure that messages are properly cleaned up.
Note that some newlines that were added to the commit message were
removed so that if a file were read via -F, it would be copied
faithfully.

Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>