Merge branch 'rs/parse-options-negation-help'

Merge branch 'rs/parse-options-negation-help'

"git cmd -h" learned to signal which options can be negated by
listing such options like "--[no-]opt".

* rs/parse-options-negation-help:
  parse-options: simplify usage_padding()
  parse-options: no --[no-]no-...
  parse-options: factor out usage_indent() and usage_padding()
  parse-options: show negatability of options in short help
  t1502: test option negation
  t1502: move optionspec help output to a file
  t1502, docs: disallow --no-help
  subtree: disallow --no-{help,quiet,debug,branch,message}