Previously, the test was passing because the Python reference implementation
of HGitaly relies on
scmutil.revsingle for revisions not found in GitLab
state files, including tags given in simple form (e.g.
But RHGitaly will rely on the GitLab state files only, hence we need to ensure that the tags state file contains the one we introduce, which is shadowing in the short SHA case
The added GitLab sync has the effect of removing directly set tags that have
no Mercurial counterpart from the GitLab state file, that's why it has to occur
write_ref() in this test of collision/shadowing were Mercurial and Git
tags must have different names.