config: add a function to insert non-file based, but overridable settings This will be used in the next patch. Until relatively recently (473510bf0575), there was no official way for extensions to inject per-repo config data, so it probably makes sense that `ui.setconfig()` items are sticky, and not affected by loading more config files. But that makes it cumbersome if the extension wants to allow the data it might add to be overridden by any data in the local hgrc file. The only thing I could get to work was to load the local hgrc first, and then check if the source for the config item that should be overridden was *not* the local hgrc file name. But that's brittle because in addition to the file name, the source contains the line number, there are the usual '\' vs '/' platform differences, etc. Differential Revision: https://phab.mercurial-scm.org/D7933