git-cl: Fix split branch name

Bug: 998922, 1054888
Change-Id: I4ae14700cbd6eb483f318f6ed9f4d1f30b58153e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2068492
Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

split_cl.py

@@ -50,7 +50,7 @@ def CreateBranchForDirectory(prefix, cl_index, directory, upstream):
   the created branch.
   """
   existing_branches = set(git.branches(use_limit = False))
-  branch_name = '_'.join([prefix, cl_index, directory])
+  branch_name = '_'.join([prefix, str(cl_index), directory])
   if branch_name in existing_branches:
     return False
   git.run('checkout', '-t', upstream, '-b', branch_name)