tests: add test for updating to null revision

While working on merge.py, I realized that we don't (as far as I could
tell) have any tests for updating to the null revision with a dirty
working copy. This adds some simple tests for that.

tests/test-update-branches.t

@@ -177,6 +177,28 b' Cases are run as shown in that table, ro'
 
   $ cd ..
 
+Test updating to null revision
+
+  $ hg init null-repo
+  $ cd null-repo
+  $ echo a > a
+  $ hg add a
+  $ hg ci -m a
+  $ hg up -qC 0
+  $ echo b > b
+  $ hg add b
+  $ hg up null
+  0 files updated, 0 files merged, 1 files removed, 0 files unresolved
+  $ hg st
+  A b
+  $ hg up -q 0
+  $ hg st
+  A b
+  $ hg up -qC null
+  $ hg st
+  ? b
+  $ cd ..
+
 Test updating with closed head
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