repos: git: Always replicate refs/ into local branches on checkout

If a refspec addresses an upstream branch which commit happens to be
local already, checkout will not properly replicate that into a local
branch. Extend contains_refspec_cmd so that it does not falsely report
the remotes/origin/ ref as locally available and rather triggers its
fetching.

The related test case was papering over this issue so far. Adjust it
accordingly.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

tests/test_refspec.py

@@ -78,8 +78,8 @@ def test_refspec_absolute(changedir, tmpdir):
     kas.kas(['shell', 'test3.yml', '-c', 'true'])
     (rc, output) = run_cmd(['git', 'symbolic-ref', '-q', 'HEAD'],
                            cwd='kas_abs', fail=False, liveupdate=False)
-    assert rc != 0
-    assert output.strip() == ''
+    assert rc == 0
+    assert output.strip() == 'refs/heads/master'
     (rc, output_kas_abs) = run_cmd(['git', 'rev-parse', 'HEAD'],
                                    cwd='kas_abs', fail=False, liveupdate=False)
     assert rc == 0