rebase -X: do not clobber strategy

rebase -X: do not clobber strategy

If any strategy options are passed to -X, the strategy will always be
set to 'recursive'. According to the documentation, it should default to
'recursive' if it is not set, but it should be possible to set it to
other values.

This fixes a regression introduced in v1.7.3-rc0~67^2 (2010-07-29).

Signed-off-by: Martin von Zweigbergk <martin.von.zweigbergk@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>