git-revert: make --edit default.

git-revert: make --edit default.

Revert always should explain why, so make --edit the default,
unless stdin is not a terminal.  If you really don't want to say
anything, you can say "git-revert --no-edit $commit", or if you
are really sick, you could also say "git-revert $commit </dev/null".
But please don't.

You can also say "git-cherry-pick --edit $commit".  Not editting
the commit log message is the default for cherry-pick.

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