[#6218] Get branches and tags directly from SCM

Signed-off-by: Cory Johns <cjohns@slashdotmedia.com>
[#6218] Fixed failing test due to added tag

Signed-off-by: Cory Johns <cjohns@slashdotmedia.com>
[#6218] Deprecated and removed references to cached heads, branches and tags

Signed-off-by: Cory Johns <cjohns@slashdotmedia.com>
[#6218] Added docstring to get_heads/branches/tags explaining why it's not a property

Signed-off-by: Cory Johns <cjohns@slashdotmedia.com>