config: drop git_config_get_string_const()

As evidenced by the leak fixes in the previous commit, the "const" in
git_config_get_string_const() clearly misleads people into thinking that
it does not allocate a copy of the string. We can fix this by renaming
it, but it's easier still to just drop it. Of the four remaining
callers:

  - The one in git_config_parse_expiry() still needs to allocate, since
    that's what its callers expect. We can just use the non-const
    version and cast our pointer. Slightly ugly, but the damage is
    contained in one spot.

  - The two in apply are writing to global "const char *" variables, and
    need to continue allocating. We often mark these as const because we
    assign default string literals to them. But in this case we don't do
    that, so we can just declare them as real "char *" pointers and use
    the non-const version.

  - The call in checkout doesn't actually need a copy; it can just use
    the non-allocating "tmp" version of the function.

The function is also mentioned in the MyFirstContribution document. We
can swap that call out for the non-allocating "tmp" variant, which fits
well in the example given.

We'll drop the "configset" and "repo" variants, as well (which are
unused).

Note that this frees up the "const" name, so we could rename the "tmp"
variant back to that. But let's give some time for topics in flight to
adapt to the new code before doing so (if we do it too soon, the
function semantics will change but the compiler won't alert us).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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