Arcyd-tester: push_new_review_branch does not handles slashes in filenames

[cs-shadow]

push_new_review_branch() writes a file corresponding to each branch and the name of the file is obtained simply by appending the name of branch to the filepath. This logic fails when the name of branch itself has slashes which is a valid case because the function treats it as a directory and hence fails while trying to create the file.

This is a relatively minor issue since this code is just used by the tester and not actually by Arcyd but thought it will still be nice to at least have it documented here.