Bence Ferdinandy [off-list ref] writes:

Consider the bare repository called "mirror" in the test.  Running `git
remote add --mirror -f origin ../one` will not change HEAD, consequently
if init.defaultBranch is not the same as what HEAD in the remote
("one"), HEAD in "mirror" will be pointing to a non-existent reference.
Hence if "mirror" is used as a remote by yet another repository,
ls-remote will not show HEAD. On the other hand, if init.defaultBranch
happens to match HEAD in "one", then ls-remote will show HEAD.

Making sure that the hardcoded (in the Git binary) default branch
name would not affect the outcome of the test is a good thing to do.
I like the patch text, but ...

Since the CI globally exports GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME=main,
there's a drift between how the test repositories are set up in the CI
and during local testing. This issue does not manifest currently, as the
test does not do any remote HEAD manipulation where this would come up,
but should such things be added, a locally passing test would break the
CI vice-versa.

... this description may not quite be accurate.  Don't some jobs of
CI use 'main' while the rest use 'master'?