Better dev defaults for fetch variable
The problem we have is if you set a branch name as the git_hash, git_clones will not fetch and update if the branch already exists (i.e. from a previous build attempt).
I propose setting testbuild/fetch to 1 by default (similar to nightly). Alternatively, we could do so just for the firefox+geckoview projects since these are the ones we are (most) actively developing.