gittest: Configure default branch to be master

Many tests depend on the fact "master" is the initial branch. To
overcome the case when the user has set anything else in their
git-config, configure git in tests to have "master" as the default
branch.

This change was introduced before in [1], but was unintentionally
removed in [2].

1. 60a21811 (testhelper: Git init with master as default branch,
   2021-04-20)
2. c0f88d78 (MustRunCommand should not be used for git commands
   execution, 2021-05-11)