CXX-3099 perform refname:strip=2 within Python (#1392)

Author: eramongodb

etc/calc_release_version.py

@@ -250,10 +250,14 @@ def iter_tag_lines():
     the second is a tag that is associated with that commit. Duplicate commits
     are possible.
     """
-    output = check_output(['git', 'for-each-ref', '--format=%(*objectname)|%(objectname)|%(refname:strip=2)', 'refs/tags/*'])
+    output = check_output(['git', 'for-each-ref', '--format=%(*objectname)|%(objectname)|%(refname)', 'refs/tags/*'])
     lines = output.splitlines()
     for l in lines:
-        obj, tagobj, tag = l.split('|', 2)
+        obj, tagobj, rawtag = l.split('|', 2)
+
+        # Support Git 1.x which does not have `%(refname:lstrip=2)`.
+        tag = str('/').join(rawtag.split('/')[:2])
+
         if not tag.startswith('r'):
             continue # We only care about "rX.Y.Z" release tags.
         if re.match(r'r\d+\.\d+', tag):