mmake help doesn't output comments from include targets

@tj , thanks for this awesome tooling.
I'm not sure if this was ever possible. But in `1.3.0` `mmake help` does not output any help data for a simple included common makefile.

### Makefile
```
include ../common.mk
```

### common.mk
```
# example command help string
start:
  @echo test
.PHONY: start
```