Re: [PATCH 03/19] global: convert intentionally-leaking config strings to consts

> There are multiple cases where we intentionally leak config strings:
>
>   - `struct gpg_format` is used to track programs that can be used for
>     signing commits, either via gpg(1), gpgsm(1) or ssh-keygen(1). The
>     user can override the commands via several config variables. As the
>     array is populated once, only, and will never be free'd, it is fine
>     to treat the program as a quasi-constant.
>
>   - `struct ll_merge_driver` is used to track merge drivers. Same as
>     with the GPG format, these drivers are populated once and then
>     reused. Its data is never free'd, either.
>
>   - `struct userdiff_funcname` and `struct userdiff_driver` can be
>     configured via `diff.<driver>.*` to add additional drivers. Again,
>     these have a global lifetime and are never free'd.
>
> All of these are intentionally kept alive and never free'd. Let's mark
> the respective fields as `const char *` and cast away the constness when
> assigning those values.