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