perf: let's disable symlinks when they are not available

We already have a perfectly fine prereq to tell us whether it is safe to use symlinks. So let's use it. This fixes the performance tests in Git for Windows' SDK, where symlinks are not really available ([*1*]). This is not an issue with Git for Windows itself because it configures core.symlinks=false in its system config. However, the system config is disabled for the performance tests, for obvious reasons: we want them to be independent of the vagaries of any local configuration. Footnote *1*: Windows has symbolic links. Git for Windows disables them by default, though (for example: in standard setups, non-admins lack the privilege to create symbolic links). For details, see https://github.com/git-for-windows/git/wiki/Symbolic-Links Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>