remote-hg: add support for schemes extension

So that we can use shortened URLs, for example 'bb:://felipec/repo'
(Bitbucket).

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/remote-helpers/git-remote-hg

@@ -12,7 +12,7 @@
 # For remote repositories a local clone is stored in
 # "$GIT_DIR/hg/origin/clone/.hg/".
 
-from mercurial import hg, ui, bookmarks, context, util, encoding, node, error
+from mercurial import hg, ui, bookmarks, context, util, encoding, node, error, extensions
 
 import re
 import sys
@@ -305,6 +305,12 @@ def get_repo(url, alias):
     except subprocess.CalledProcessError:
         pass
 
+    try:
+        mod = extensions.load(myui, 'hgext.schemes', None)
+        mod.extsetup(myui)
+    except ImportError:
+        pass
+
     if hg.islocal(url):
         repo = hg.repository(myui, url)
     else: