diff -r 6fa245f80b6f -r 9fcb6df413c9 mercurial/ui.py
--- a/mercurial/ui.py	Thu Jun 15 15:13:18 2017 -0700
+++ b/mercurial/ui.py	Wed Jun 14 20:56:34 2017 -0400
@@ -43,6 +43,20 @@
 _keepalnum = ''.join(c for c in map(pycompat.bytechr, range(256))
                      if not c.isalnum())
 
+# The config knobs that will be altered (if unset) by ui.tweakdefaults.
+tweakrc = """
+[ui]
+# The rollback command is dangerous. As a rule, don't use it.
+rollback = False
+
+[commands]
+# Make `hg status` emit cwd-relative paths by default.
+status.relative = yes
+
+[diff]
+git = 1
+"""
+
 samplehgrcs = {
     'user':
 """# example user config (see 'hg help config' for more info)
@@ -182,6 +196,7 @@
             self.fin = src.fin
             self.pageractive = src.pageractive
             self._disablepager = src._disablepager
+            self._tweaked = src._tweaked
 
             self._tcfg = src._tcfg.copy()
             self._ucfg = src._ucfg.copy()
@@ -205,6 +220,7 @@
             self.fin = util.stdin
             self.pageractive = False
             self._disablepager = False
+            self._tweaked = False
 
             # shared read-only environment
             self.environ = encoding.environ
@@ -241,8 +257,29 @@
                     u.fixconfig(section=section)
             else:
                 raise error.ProgrammingError('unknown rctype: %s' % t)
+        u._maybetweakdefaults()
         return u
 
+    def _maybetweakdefaults(self):
+        if not self.configbool('ui', 'tweakdefaults'):
+            return
+        if self._tweaked or self.plain('tweakdefaults'):
+            return
+
+        # Note: it is SUPER IMPORTANT that you set self._tweaked to
+        # True *before* any calls to setconfig(), otherwise you'll get
+        # infinite recursion between setconfig and this method.
+        #
+        # TODO: We should extract an inner method in setconfig() to
+        # avoid this weirdness.
+        self._tweaked = True
+        tmpcfg = config.config()
+        tmpcfg.parse('<tweakdefaults>', tweakrc)
+        for section in tmpcfg:
+            for name, value in tmpcfg.items(section):
+                if not self.hasconfig(section, name):
+                    self.setconfig(section, name, value, "<tweakdefaults>")
+
     def copy(self):
         return self.__class__(self)
 
@@ -387,6 +424,7 @@
         for cfg in (self._ocfg, self._tcfg, self._ucfg):
             cfg.set(section, name, value, source)
         self.fixconfig(section=section)
+        self._maybetweakdefaults()
 
     def _data(self, untrusted):
         return untrusted and self._ucfg or self._tcfg