src/bst_plugins_experimental/testutils/repo/gitrepo.py: Use git_tag

Tristan Van Berkomrequested to merge
tristan/fix-source-tests into master

This repo is intended to test the git_tag source from bst-plugins-experimental but was accidentally testing the upstream git source, which is going to be removed from upstream core plugins.

This will cause tests to pass after upstream removes the git source, and also cause the git_tag source to actually get tested with the standardized tests.

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