rebase -p: fix incorrect commit message when calling `git merge`.

rebase -p: fix incorrect commit message when calling `git merge`.

Since commit dd6fb0053 ("rebase -p: fix quoting when calling `git
merge`"), commit message of the merge commit being rebased is passed to
the merge command using a subshell executing 'git rev-parse --sq-quote'.

Double quotes are needed around this subshell so that, newlines are
kept for the git merge command.

Before this patch, following merge message:

    "Merge mybranch into mynewbranch

    Awesome commit."

becomes:

    "Merge mybranch into mynewbranch Awesome commit."

after a rebase -p.

Fixes: "dd6fb0053 rebase -p: fix quoting when calling `git merge`"
Reported-by: Jamie Iles <jamie.iles@oracle.com>
Suggested-by: Vegard Nossum <vegard.nossum@oracle.com>
Suggested-by: Quentin Casasnovas <quentin.casasnovas@oracle.com>
Signed-off-by: Gregory Herrero <gregory.herrero@oracle.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>