flag_override() may not always restore the original flags

[odelalleau]

Describe the bug

When restoring flags, flag_override() may accidentally restore flag inherited from the parent:

c = OmegaConf.create({"a": {"b": 1}})
print(f"Before: {c.a._get_flag('test')}")
with flag_override(c, "test", True):
    with flag_override(c.a, "test", False):
        print(f"During: {c.a._get_flag('test')}")
print(f"After: {c.a._get_flag('test')}")

Output:

Before: None
During: False
After: True  <== should be `None`!

[omry]

nice catch. #590.