Merge branch 'ab/help-config-vars'

Merge branch 'ab/help-config-vars'

Teach "git help -c" into helping the command line completion of
configuration variables.

* ab/help-config-vars:
  help: move column config discovery to help.c library
  help / completion: make "git help" do the hard work
  help tests: test --config-for-completion option & output
  help: simplify by moving to OPT_CMDMODE()
  help: correct logic error in combining --all and --guides
  help: correct logic error in combining --all and --config
  help tests: add test for --config output
  help: correct usage & behavior of "git help --guides"
  help: correct the usage string in -h and documentation