Every mkrep call in this suite runs a bare `git init`. On a runner with
no init.defaultBranch configured, git prints its "Using 'master' as the
name for the initial branch" advice block on each one (20 occurrences in
CI's test-job log, all from this file). test-agents-vault.fish already
pins the same setting via GIT_CONFIG_COUNT/KEY/VALUE for the identical
reason; apply the same fix here rather than in mkrep.fish itself, so a
real user's own git config still wins in normal use.