Set 'fetch: 1' by default for nightly builds
In tor-browser-build, when doing some build, we fetch new commits/branchs/tags from a git repository when the git revision specified in git_hash does not exist in the local repository. If the revision already exist, we don't update the repository. This means that if git_hash is pointing to a branch like master and the master branch already exists, it won't be updated. This can be confusing for the nightly builds, where you have to remember to run make fetch first if you want to build the very latest nightly, and not a nightly based on the commits from the last make fetch.
To avoid that we can set fetch: 1 for nightly in rbm.conf so that git repositories are updated for each build by default. People who don't want that can still change it in their rbm.local.conf.