[REGRESSION] git var GIT_EDITOR fails without tty

Hello,

since b4479f074760a788dd4e353b8c86a7d735afc53e git send-email (and others) use git var GIT_EDITOR. This is OK as such but it breaks the post-receive hooks that I use on several repositories.

When called they don't have a tty, and this makes git var fail:

	user@host:~$ ssh localhost "git var GIT_EDITOR"
	fatal: Terminal is dumb, but EDITOR unset

IMHO git send-email should only call $(git var GIT_EDITOR) when it actually needs it.

(Note I'm using the Debian packaged version of git. I don't think this problem is Debian specific, I didn't check though.)

Best regards Uwe