more care about parameter immutable in sage/graphs/generic_graph.py

[dcoudert]

Following #39280, #39285, #39287, #39296 and discussions in #39177, we add parameter immutable to methods in sage/graphs/generic_graph.py:

• longest_cycle
• longest_path
• hamiltonian_path
• and add tests in cycle_basis

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[kwankyu]

Namewise, mutable=True (default) would have been a better choice. But I also believe immutable was chosen after all things considered by experts.

[kwankyu]

LGTM.