Tests: Fix CTest.UpdateGIT under repo-local defaultBranch config

The default branch detection added by commit 26ec2e2b0c (Tests: Update
CTest.UpdateGIT test for custom defaultBranch, 2020-12-02,
v3.20.0-rc1~355^2) fails if the test is run inside a Git work tree whose
`.git` directory configures a `defaultBranch` that is different from the
global or system-wide value.  Fix the detection to ignore the locally
configured value so that we match the `git init` decision.