t9117: use --prefix "" instead of --prefix=""

t9117: use --prefix "" instead of --prefix=""

Versions of Perl's Getopt::Long module before 2.37 do not contain
this fix that first appeared in Getopt::Long version 2.37:

* Bugfix: With gnu_compat, --foo= will no longer trigger "Option
  requires an argument" but return the empty string.

Instead of using --prefix="" use --prefix "" when testing an
explictly empty prefix string in order to work with older versions
of Perl's Getopt::Long module.

Also add a paragraph on this workaround to the documentation of
git-svn itself.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>
Acked-by: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>