git-cvsimport-3: remove unused parameter to *_or_die

The legend parameters to do_or_die() and capture_or_die() are never
used.  Remove them.

Signed-off-by: John Keeping <john@keeping.me.uk>
git-cvsimport-3: handle the backend option first

Most of the options to git-cvsimport-3 are simply passed to the backend,
this means the if the '-e <backend>' option does not occur first any
options preceding it will effectively be ignored.

Extract the '-e' option handling as a special case before the main
option handling loop to avoid this issue.

Signed-off-by: John Keeping <john@keeping.me.uk>