subrepo: use hg.repository instead of creating localrepo directly
this way, extensions' reposetup will be called, which allows for git
subrepos to be handled by hg-git (and I believe the same goes for
svn and hgsubversion)
% init
adding a
% fold in the middle of the queue
now at: p1
--- a/a
+++ b/a
@@ -1,1 +1,2 @@
a
+a
p1
p3
--- a/a
+++ b/a
@@ -1,1 +1,3 @@
a
+a
+b
% fold with local changes
abort: local changes found, refresh first
--- a/a
+++ b/a
@@ -1,1 +1,3 @@
a
+a
+b