fix: loosen lifetime constraint on mut_subcommand

[emersonford]

follow-up to PR #3882, as discussed in that PR's comments mut_subcommand does not resolve aliases and, if an alias name is passed for subcmd_id, would trigger invalid behavior as per #3888.

thus to avoid this, one might resolve the alias name to the actual name before calling mut_subcommand, but this is currently almost impossible to do due to mut_subcommand signature's with the 'help lifetime. specifically, looking at the test code included in this PR and without the lifetime changes, we get the following error:

error[E0597]: `true_name` does not live long enough
   --> tests/builder/tests.rs:459:32
    |
459 |     cmd = cmd.mut_subcommand(&*true_name, |subcmd| subcmd.about("modified about"));
    |                                ^^^^^^^^^ borrowed value does not live long enough
460 |     assert_eq!(cmd.find_subcommand("baz").unwrap().get_about().unwrap(), "modified about");
461 | }
    | -
    | |
    | `true_name` dropped here while still borrowed
    | borrow might be used here, when `cmd` is dropped and runs the destructor for type `App<'_>`
    |
    = note: values in a scope are dropped in the opposite order they are defined

For more information about this error, try `rustc --explain E0597`.
error: could not compile `clap` due to previous error

but loosening the lifetime constraint from 'help to 'a resolves this.

note, directly using cmd.find_subcommand("baz").unwrap().get_name() is invalid because of:

error[E0597]: `cmd` does not live long enough
   --> tests/builder/tests.rs:457:21
    |
457 |     let true_name = cmd.find_subcommand("baz").unwrap().get_name();
    |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^ borrowed value does not live long enough
...
465 | }
    | -
    | |
    | `cmd` dropped here while still borrowed
    | borrow might be used here, when `cmd` is dropped and runs the destructor for type `App<'_>`

error[E0506]: cannot assign to `cmd` because it is borrowed
   --> tests/builder/tests.rs:460:5
    |
457 |     let true_name = cmd.find_subcommand("baz").unwrap().get_name();
    |                     -------------------------- borrow of `cmd` occurs here
...
460 |     cmd = cmd.mut_subcommand(true_name, |subcmd| subcmd.about("modified about"));
    |     ^^^
    |     |
    |     assignment to borrowed `cmd` occurs here
    |     borrow later used here

error[E0505]: cannot move out of `cmd` because it is borrowed
   --> tests/builder/tests.rs:460:11
    |
457 |     let true_name = cmd.find_subcommand("baz").unwrap().get_name();
    |                     -------------------------- borrow of `cmd` occurs here
...
460 |     cmd = cmd.mut_subcommand(true_name, |subcmd| subcmd.about("modified about"));
    |           ^^^                --------- borrow later used here
    |           |
    |           move out of `cmd` occurs here

Some errors have detailed explanations: E0505, E0506, E0597.
For more information about an error, try `rustc --explain E0505`.
error: could not compile `clap` due to 3 previous errors

I'd also like to do this for mut_arg, but that's a more involved change so will put that in a separate PR. :)

[epage]

I'd also like to do this for mut_arg, but that's a more involved change so will put that in a separate PR. :)

We'll also be removing all of the lifetimes in 4.0, so the problem will be minimized with time.