Merge branch 'ps/builtin-config-cleanup'

Code clean-up to reduce inter-function communication inside
builtin/config.c done via the use of global variables.

* ps/builtin-config-cleanup: (21 commits)
  builtin/config: pass data between callbacks via local variables
  builtin/config: convert flags to a local variable
  builtin/config: track "fixed value" option via flags only
  builtin/config: convert `key` to a local variable
  builtin/config: convert `key_regexp` to a local variable
  builtin/config: convert `regexp` to a local variable
  builtin/config: convert `value_pattern` to a local variable
  builtin/config: convert `do_not_match` to a local variable
  builtin/config: move `respect_includes_opt` into location options
  builtin/config: move default value into display options
  builtin/config: move type options into display options
  builtin/config: move display options into local variables
  builtin/config: move location options into local variables
  builtin/config: refactor functions to have common exit paths
  config: make the config source const
  builtin/config: check for writeability after source is set up
  builtin/config: move actions into `cmd_config_actions()`
  builtin/config: move legacy options into `cmd_config()`
  builtin/config: move subcommand options into `cmd_config()`
  builtin/config: move legacy mode into its own function
  ...

Author: gitster