Merge branch 'jk/config-include'

Merge branch 'jk/config-include'

* jk/config-include:
  : An assignment to the include.path pseudo-variable causes the named file
  : to be included in-place when Git looks up configuration variables.
  config: add include directive
  config: eliminate config_exclusive_filename
  config: stop using config_exclusive_filename
  config: provide a version of git_config with more options
  config: teach git_config_rename_section a file argument
  config: teach git_config_set_multivar_in_file a default path
  config: copy the return value of prefix_filename
  t1300: add missing &&-chaining
  docs/api-config: minor clarifications
  docs: add a basic description of the config API