Dependency loops cause targets to be missing from ninja graph with --gen_all_targets

[danw]

(From internal google bug 162918277)

This test case can show the problem:

testcase/circular_dep_self.mk

test:
	echo PASS

loop: loop test
	true

$ go test --ckati --ninja --all -test.run=TestKati/circular_dep_self.mk
--- FAIL: TestKati (0.00s)
    --- FAIL: TestKati/phony_loop.mk (0.02s)
        run_test.go:484: Different output from kati (red) to the expected value from make (green):
            src/ninja.cc:671: default_target_PASS

Ninja is erroring out because there are no default targets defined:

# Generated by kati ...

pool local_pool
 depth = 72

build _kati_always_build_: phony

Removing the circular dependency causes things to work:

# Generated by kati ...

pool local_pool
 depth = 72

build _kati_always_build_: phony

rule rule0
 description = build $out
 command = /bin/bash -c "true"
build loop: rule0 test
rule rule1
 description = build $out
 command = /bin/bash -c "echo PASS"
build test: rule1

default test

[danw]

This is due to the calculation of non_root_targets here. We could handle the self-referential loop rather easily by comparing p.first/t, but deeper loops would be more difficult to detect and handle with the current code.