give a better error for "multiple rules generate <file>"

[jyn514]

right now, if there are two duplicate rules in a build.ninja file, ninja prints the following:

ninja: error: build.ninja:238: multiple rules generate .build/talks/python.rendered.html

ideally it would give more context: which rules? what are their inputs? i can't figure either out easily because -t inputs and -t query don't work when ninja can't parse the depfile.

ideally i would love an error message like this:

ninja: error: build.ninja:238: multiple rules generate .build/talks/python.rendered.html
  build.ninja:237: build .build/talks/python.rendered.html: markdown .build/talks/python.rendered.md
      build.ninja:236: build .build/talks/python.rendered.md: page .build/talks/python.md.json
        build.ninja:235: build .build/talks/python.md.json: frontmatter pages/talks/python.md
  build.ninja:98: build .build/talks/python.rendered.html: page .build/talks/python.html.json
      build.ninja:97: build .build/talks/python.html.json: frontmatter pages/talks/python.html

(note that this omits implicit and order-only dependencies).

if that's too expensive to store all the data for in a first run, maybe it could be hidden behind --verbose, and have the error suggest using verbose for more info?