test: isolate tests from system/global git config

go-git's default NewAuto ConfigLoader and the git binary both read the host's global/system config, leaking settings like commit.gpgSign=true into tests. Add syncertest.IsolateGitConfig, which points GIT_CONFIG_GLOBAL/SYSTEM at os.DevNull, and call it from each affected TestMain. This replaces the syncer package's go-git-only plugin override, which left the SSH integration test still reading the host config.