Add test for branch shadowing

tests/test-newbranch

@@ -12,7 +12,9 b' hg branch'
 hg ci -m "add branch name" -d "1000000 0"
 hg branch bar
 hg ci -m "change branch name" -d "1000000 0"
+echo % branch shadowing
 hg branch ""
+hg branch -f ''
 hg ci -m "clear branch name" -d "1000000 0"
 
 hg co foo

tests/test-newbranch.out

@@ -1,4 +1,6 b''
 foo
+% branch shadowing
+abort: a branch of the same name already exists (use --force to override)
 0 files updated, 0 files merged, 0 files removed, 0 files unresolved
 foo
 0 files updated, 0 files merged, 0 files removed, 0 files unresolved