prepare_repo_settings(): plug leak of config values

We call repo_config_get_string() for fetch.negotiationAlgorithm, which allocates a copy of the string, but we never free it. We could add a call to free(), but there's an even simpler solution: we don't need the string to persist beyond a few strcasecmp() calls, so we can instead use the "_tmp" variant which gives us a const pointer to the cached value. We need to switch the type of "strval" to "const char *" for this to work, which affects a similar call that checks core.untrackedCache. But it's in the same boat! It doesn't actually need the value to persist beyond a maybe_bool() check (though it does remember to correctly free the string afterwards). So we can simplify it at the same time. Note that this core.untrackedCache check arguably should be using repo_config_get_maybe_bool(), but there are some subtle behavior changes. E.g., it doesn't currently allow a value-less "true". Arguably it should, but let's avoid lumping further changes in what should be a simple leak cleanup. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>