Currently, our default branch in tests is "master". This is the Git
default, but the Git default will likely change in the future, so it
makes sense to update our testsuite to be explicit about the branch
name. We'll ensure this continues by building against older versions of
Git as well as newer versions.
We use "main" for the new branch name, since that's the proposed value
upstream.
This commit was made entirely by automated means using the following
command:
git grep -l master t | xargs sed -i -e 's/master/main/g'
