builtin/help: fix leaks in `check_git_cmd()`

builtin/help: fix leaks in `check_git_cmd()`

The `check_git_cmd()` function is declared to return a string constant.
And while it sometimes does return a constant, it may also return an
allocated string in two cases:

  - When handling aliases. This case is already marked with `UNLEAK()`
    to work around the leak.

  - When handling unknown commands in case "help.autocorrect" is
    enabled. This one is not marked with `UNLEAK()`.

The function only has a single caller, so let's fix its return type to
be non-constant, consistently return an allocated string and free it at
its callsite to plug the leak.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>