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

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>