config: introduce "built-in" default configuration settings in default.d

This helps providing a more consistent user experience on all platforms and
with all packaging.

The exact location of default.d depends on how Mercurial is installed and
whether it is 'frozen'. The exact location should never be relevant to users
and is intentionally not explained in details in the documentation. It will
however always be next to the help and templates files.

Note that setting HGRCPATH also disables these defaults. I don't know if that
should be considered a bug or a feature.

mercurial/help/config.txt

@@ -38,6 +38,7 b' ones.'
   - ``<install-root>/etc/mercurial/hgrc.d/*.rc`` (per-installation)
   - ``/etc/mercurial/hgrc`` (per-system)
   - ``/etc/mercurial/hgrc.d/*.rc`` (per-system)
+  - ``<internal>/default.d/*.rc`` (defaults)
 
 .. container:: verbose.windows
 
@@ -51,6 +52,7 b' ones.'
   - ``<install-dir>\Mercurial.ini`` (per-installation)
   - ``<install-dir>\hgrc.d\*.rc`` (per-installation)
   - ``HKEY_LOCAL_MACHINE\SOFTWARE\Mercurial`` (per-installation)
+  - ``<internal>/default.d/*.rc`` (defaults)
 
   .. note::
 
@@ -67,6 +69,7 b' ones.'
   - ``<install-root>/lib/mercurial/hgrc.d/*.rc`` (per-installation)
   - ``/lib/mercurial/hgrc`` (per-system)
   - ``/lib/mercurial/hgrc.d/*.rc`` (per-system)
+  - ``<internal>/default.d/*.rc`` (defaults)
 
 Per-repository configuration options only apply in a
 particular repository. This file is not version-controlled, and
@@ -102,6 +105,13 b' is running. Options in these files apply'
 executed by any user in any directory. Options in these files
 override per-installation options.
 
+Mercurial comes with some default configuration. The default configuration
+files are installed with Mercurial and will be overwritten on upgrades. Default
+configuration files should never be edited by users or administrators but can
+be overridden in other configuration files. So far the directory only contains
+merge tool configuration but packagers can also put other default configuration
+there.
+
 Syntax
 ======
 

mercurial/scmutil.py

@@ -495,7 +495,13 b' def walkrepos(path, followsym=False, see'
 
 def osrcpath():
     '''return default os-specific hgrc search path'''
-    path = systemrcpath()
+    path = []
+    defaultpath = os.path.join(util.datapath, 'default.d')
+    if os.path.isdir(defaultpath):
+        for f, kind in osutil.listdir(defaultpath):
+            if f.endswith('.rc'):
+                path.append(os.path.join(defaultpath, f))
+    path.extend(systemrcpath())
     path.extend(userrcpath())
     path = [os.path.normpath(f) for f in path]
     return path

setup.py

@@ -517,6 +517,7 b' cygwinccompiler.Mingw32CCompiler = Hacke'
 
 packagedata = {'mercurial': ['locale/*/LC_MESSAGES/hg.mo',
                              'help/*.txt',
+                             'default.d/*.rc',
                              'dummycert.pem']}
 
 def ordinarypath(p):