git-svn: On MSYS, escape and quote SVN_SSH also if set by the user

git-svn: On MSYS, escape and quote SVN_SSH also if set by the user

While GIT_SSH does not require any escaping / quoting (e.g. for paths
containing spaces), SVN_SSH requires it due to its use in a Perl script.

Previously, SVN_SSH has only been escaped and quoted automatically if it
was unset and thus derived from GIT_SSH. For user convenience, do the
escaping and quoting also for a SVN_SSH set by the user. This way, the
user is able to use the same unescaped and unquoted syntax for GIT_SSH
and SVN_SSH.

Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>