superproject: Pass branch to git ls-tree.

Tested the code with the following commands.

$ ./run_tests -v

Bug: [google internal] b/179702819
Bug: https://crbug.com/gerrit/13709
Bug: https://crbug.com/gerrit/13707


Tested-by: Raman Tenneti <rtenneti@google.com>
Change-Id: I7d2b609ac2f927c94701757aa1502ba236afe7c0
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/296342


Reviewed-by: Mike Frysinger <vapier@google.com>