When pushing with git with the two server-side git hook scripts in rbtools/contrib/tools, a Python TypeError is raised. This is due to an incorrect function call in the get_commit_hashes function in rbtools/hooks/git.py. Changing this call to return a list fixes the issue

I tested on my computer, by creating two local repositories, one pointing to the other one with the following configurations:

- git remote add <path_to_other_local_git_repo> to the pushing repo

- git config receive.denyCurrentBranch ignore to the receiving repo
I also installed the 2 server side hooks in contrib/tools on the pushing side.
The scripts work as intended after the fix.