Merge branch 'ls/config-origin'

Merge branch 'ls/config-origin'

The configuration system has been taught to phrase where it found a
bad configuration variable in a better way in its error messages.
"git config" learnt a new "--show-origin" option to indicate where
the values come from.

* ls/config-origin:
  config: add '--show-origin' option to print the origin of a config value
  config: add 'origin_type' to config_source struct
  rename git_config_from_buf to git_config_from_mem
  t: do not hide Git's exit code in tests using 'nul_to_q'