config: give it an includepaths option for looking for config files

It is desirable to "derive" templates from the provided templates. A
simple way to do this is e.g.

    %include map-cmdline.default

in your own mapfile. Then you only have to redefine a few templates
instead of copying over the whole thing. This %include mechanism
already works for the built-in templates because by default it *only*
looks for files that are in the same directory as the including
mapfile.

With this changeset, config grows an option to add more include paths
for config files.

mercurial/config.py

@@ -10,10 +10,11 b' import error, util'
 import os, errno
 
 class config(object):
-    def __init__(self, data=None):
+    def __init__(self, data=None, includepaths=[]):
         self._data = {}
         self._source = {}
         self._unset = []
+        self._includepaths = includepaths
         if data:
             for k in data._data:
                 self._data[k] = data[k].copy()
@@ -110,13 +111,17 b' class config(object):'
                 item = None
                 cont = False
             m = includere.match(l)
-            if m:
-                inc = util.expandpath(m.group(1))
-                base = os.path.dirname(src)
-                inc = os.path.normpath(os.path.join(base, inc))
-                if include:
+
+            if m and include:
+                expanded = util.expandpath(m.group(1))
+                includepaths = [os.path.dirname(src)] + self._includepaths
+
+                for base in includepaths:
+                    inc = os.path.normpath(os.path.join(base, expanded))
+
                     try:
                         include(inc, remap=remap, sections=sections)
+                        break
                     except IOError, inst:
                         if inst.errno != errno.ENOENT:
                             raise error.ParseError(_("cannot include %s (%s)")