tests: ignore git's output in test-subrepo-git.t

git version 1.7.9.1 makes the testsuite fail with:

@@ -482,7 +482,17 @@

 Sticky repository, update --clean
   $ hg update --clean tip
-  Previous HEAD position was aa84837... f
+  Warning: you are leaving 2 commits behind, not connected to
+  any of your branches:
+
+    aa84837 f
+    126f2a1 gg
+
+  If you want to keep them by creating a new branch, this may be a good time
+  to do so with:
+
+   git branch new_branch_name aa84837ccfbdfedcdcdeeedc309d73e6eb069edc
+
   HEAD is now at 32a3438... fff
   1 files updated, 0 files merged, 0 files removed, 0 files unresolved
   $ hg id -n

ERROR: test-subrepo-git.t output changed

tests/test-subrepo-git.t

@@ -481,9 +481,7 b' Sticky subrepository, file changes and r'
   $ cd ..
 
 Sticky repository, update --clean
-  $ hg update --clean tip
-  Previous HEAD position was aa84837... f
-  HEAD is now at 32a3438... fff
+  $ hg update --clean tip 2>/dev/null
   1 files updated, 0 files merged, 0 files removed, 0 files unresolved
   $ hg id -n
   7