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