[Bug libstdc++/118293] Inserting at front of an empty deque shouldn't need to allocate

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118293

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Similarly:

    using V = std::deque<ValueType, A<ValueType>>;
    V v;
    printf("Default-constructed capacity: %zu\n", capacity_of_v());
    v.assign(capacity_of_v(), 1);
    printf("Capacity after assign: %zu\n", capacity_of_v());

gives:

Default-constructed capacity: 128
Capacity after assign: 256

Fixing it in _M_reserve_elements_at_front would help this case too.