Merge branch 'ps/config-subcommands'

Merge branch 'ps/config-subcommands'

The operation mode options (like "--get") the "git config" command
uses have been deprecated and replaced with subcommands (like "git
config get").

* ps/config-subcommands:
  builtin/config: display subcommand help
  builtin/config: introduce "edit" subcommand
  builtin/config: introduce "remove-section" subcommand
  builtin/config: introduce "rename-section" subcommand
  builtin/config: introduce "unset" subcommand
  builtin/config: introduce "set" subcommand
  builtin/config: introduce "get" subcommand
  builtin/config: introduce "list" subcommand
  builtin/config: pull out function to handle `--null`
  builtin/config: pull out function to handle config location
  builtin/config: use `OPT_CMDMODE()` to specify modes
  builtin/config: move "fixed-value" option to correct group
  builtin/config: move option array around
  config: clarify memory ownership when preparing comment strings