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

mercurial/ui.py

@@ -467,6 +467,25 b' class ui(object):'
                     raise
                 self.warn(_(b'ignored: %s\n') % stringutil.forcebytestr(inst))
 
+        self._applyconfig(cfg, trusted, root)
+
+    def applyconfig(self, configitems, source=b"", root=None):
+        """Add configitems from a non-file source.  Unlike with ``setconfig()``,
+        they can be overridden by subsequent config file reads.  The items are
+        in the same format as ``configoverride()``, namely a dict of the
+        following structures: {(section, name) : value}
+
+        Typically this is used by extensions that inject themselves into the
+        config file load procedure by monkeypatching ``localrepo.loadhgrc()``.
+        """
+        cfg = config.config()
+
+        for (section, name), value in configitems.items():
+            cfg.set(section, name, value, source)
+
+        self._applyconfig(cfg, True, root)
+
+    def _applyconfig(self, cfg, trusted, root):
         if self.plain():
             for k in (
                 b'debug',