config: add --source option to include source of value Showing the source of each config option is quite useful and not something the user should have to reach for the `--debug` flag for. I updates documentation and tests, except for one place in `test-hgrc.t` where I thought the test might have been intended to also test that `--debug` results in `ui.quiet` etc being test. Differential Revision: https://phab.mercurial-scm.org/D10668

== New Features ==

 * `hg config` now has a `--source` option to show where each
   configuration value comes from.


== Default Format Change ==

These changes affects newly created repositories (or new clone) done with
Mercurial 5.8.


== New Experimental Features ==


== Bug Fixes ==


== Backwards Compatibility Changes ==


== Internal API Changes ==