Add test for invalid branch cache (fixed by 27ebe4efe98e)

tests/test-newbranch

@@ -23,4 +23,6 b' cat .hg/branch'
 HG_MERGE=true hg ci -m "merge" -d "0 0"
 hg log
 
-
+echo % test for invalid branch cache
+hg rollback
+hg tip

tests/test-newbranch.out

@@ -41,3 +41,13 b' user: test'
 date:        Thu Jan 01 00:00:00 1970 +0000
 summary:     initial
 
+% test for invalid branch cache
+rolling back last transaction
+changeset:   4:98d14f698afe
+branch:      foo
+tag:         tip
+parent:      1:0079f24813e2
+user:        test
+date:        Thu Jan 01 00:00:00 1970 +0000
+summary:     modify a branch
+