Omit commented-out options in `configure.py`

[jyn514]

Right now, ./configure --set rust.incremental generates an 809 line file, making it very hard to tell what's actually modified. configure-args kind of helps with this, but doesn't show up until line 331, so it's not obvious to people who aren't experienced with bootstrap. We should change configure.py to only include options that are actually set, so it's easier to tell how it differs from the defaults. This also avoids issues where the comments in config.toml drift away from the comments in config.toml.example over time.

Unfortunately config.toml.example is just a bunch of comments so it's annoying to actually do this and keep the relevant comments in config.toml ... maybe we can search back upwards to the nearest blank line?

Relevant code:

rust/src/bootstrap/configure.py

Lines 383 to 393 in 6379c72

	for line in open(rust_dir + '/config.toml.example').read().split("\n"):
	if line.startswith('['):
	cur_section = line[1:-1]
	if cur_section.startswith('target'):
	cur_section = 'target'
	elif '.' in cur_section:
	raise RuntimeError("don't know how to deal with section: {}".format(cur_section))
	sections[cur_section] = [line]
	section_order.append(cur_section)
	else:
	sections[cur_section].append(line)

[Teapot4195]

@rustbot claim