argv-parser-rewrite

[kiprasmel]

BREAKING CHANGE: completely overhaul arg parsing & allow invoking CLI w/o initialBranch

...as long as the initialBranch was already provided in some previous invocation of stacked-rebase, thus cached. sometime later, we could have a config for the default initial branch..

• create generic utils for argv parsing (separate package soon?)

• introduce multiple tests, which caught lots of (mostly new) bugs!

• tidy up code at start of gitStackedRebase, which now makes great sense & looks nice

  • same in other places

• clear up mental model of "parsing options" - it's "resolve" instead of "parse"

• move initialBranch parsing into resolveOptions

  • don't want out of sync resolvedOptions.initialBranch vs nameOfInitialBranch

• get rid of "intial branch is optional for some args" -- non issue

  • because e.g. for --continue to have any meaning, you'd have to have already started a stacked rebase & thus would have the initial branch cached.