Tweaks to make reproducing builds from crater easier

[rsheeter]

• Set defaults on crater arguments and simplify example commands
• Use the same default for local font repos in ttx_diff
• Default ttx_diff to compare default because gftools doesn't actually do what it says at time of writing
• Add the ability to run ttx_diff against a specific source in a repository
  • python resources/scripts/ttx_diff.py https://github.com/cyrealtype/Alice#sources/Alice.glyphs will check for a local copy of the repo, clone it if there isn't one, and then build/compare the glyphs file specified in the fragment
• Document how to run ci locally to get an html report; very useful when trying to alter the html!
• Add a copy repro button to the results

Copy repro gets you something like python resources/scripts/ttx_diff.py https://github.com/googlefonts/are-you-serious#sources/AreYouSerious.glyphs which ttx_diff now supports.

[cmyr]

overall looks like some nice improvements but I have concerns about the defaults, specifically ../fonts and to a lesser extent the results.json, more detail inline.

[cmyr]

Okay so to summarize a discussion from chat:

• we can just get rid of the 'fonts_repo' arg completely, it's a distraction. I've done that in [crater] Remove 'fonts_repo' cli arg #1041
• for hard-coding a default cache location I would prefer something like .fontc_crater_cache, under the user's home directory. This isn't perfect, but a relative path won't work in the CI case (since we clone the fontc repo into a new working directory and then rm -rf it after each run)