Emulate the Git for Windows `core.symlinks` support.  Since symbolic
links are generally enabled for Administrator (and _may_ be enabled due
to enabling Developer mode) but symbolic links are still sufficiently
uncommon on Windows that Git users are expected to explicitly opt-in to
symbolic links by enabling `core.symlinks=true` in a global (or xdg or
system) configuration.

When `core.symlinks=true` is set globally _and_ symbolic links support
is detected then new repositories created will not have a
`core.symlinks` set.  If `core.symlinks` is _not_ set then no detection
will be performed, and `core.symlinks=false` will be set in the
repository configuration.