Consistent message encoding while reusing log from an existing commit.

Consistent message encoding while reusing log from an existing commit.

The following commands can reuse log message from an existing
commit while creating a new commit:

	git-cherry-pick
	git-rebase (both with and without --merge)
	git-commit (-c and -C)

When the original commit was made in a different encoding from
the current i18n.commitencoding, "cat-file commit" would give a
string that is inconsistent with what the resulting commit will
claim to be in.  Replace them with "git show -s --encoding".

"git-rebase" without --merge is "git format-patch" piped to "git
am" in essence, and has been taken care of before this commit.

Signed-off-by: Junio C Hamano <junkio@cox.net>