rebase -i: abort cleanly if the editor fails to launch

If the user's configured editor is emacsclient, the editor will fail to launch if emacs is not running and the git command that tried to lanuch the editor will abort. For most commands, all you have to do is to start emacs and repeat the command. The "git rebase -i" command, however, aborts without cleaning the "$GIT_DIR/rebase-merge" directory if it fails to launch the editor, so you'll need to do "git rebase --abort" before repeating the rebase command. Change "git rebase -i" to terminate using "die_abort" (instead of with "die") if the initial launch of the editor fails. Signed-off-by: Björn Gustavsson <bgustavsson@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>