Merge branch 'po/help-guides'

"git help" learned "-g" option to show the list of guides just like
list of commands are given with "-a".

* po/help-guides:
  doc: include --guide option description for "git help"
  help: mention -a and -g option, and 'git help <concept>' usage.
  builtin/help.c: add list_common_guides_help() function
  builtin/help.c: add --guide option
  builtin/help.c: split "-a" processing into two