rebase -p: fix quoting when calling `git merge`

rebase -p: fix quoting when calling `git merge`

It has been reported that strategy arguments are not passed to `git
merge` correctly when rebasing interactively, preserving merges.

The reason is that the strategy arguments are already quoted, and then
quoted again.

This fixes https://github.com/git-for-windows/git/issues/1321

Original-patch-by: Kim Gybels <kgybels@infogroep.be>
Also-reported-by: Matwey V. Kornilov <matwey.kornilov@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>