cvsimport: Exit if an existing repository doesn't have the right branch.

git-cvsimport-script

@@ -338,6 +338,11 @@ unless(-d $git_dir) {
 	$last_branch = $opt_o;
 	$orig_branch = "";
 } else {
+	-f "$git_dir/refs/head/$opt_o"
+		or die "Branch '$opt_o' does not exist.\n".
+		       "Either use the correct '-o branch' option,\n".
+		       "or import to a new repository.\n";
+
 	$last_branch = basename(readlink("$git_dir/HEAD"));
 	unless($last_branch) {
 		warn "Cannot read the last branch name: $! -- assuming 'master'\n";