Micro-optimization to avoid creating temporary path

Micro-optimization to avoid creating temporary path

Now that path::operator/=(basic_string_view<value_type>) works directly
from the string argument, instead of constructing a temporary path from
the string, it's potentially more efficient to do 'path(x) /= s' instead
of 'x / s'. This changes the only relevant place in the library.

	* src/filesystem/std-dir.cc (filesystem::_Dir::advance): Append
	string to lvalue to avoid creating temporary path.

From-SVN: r267236