prepare_repo_settings(): plug leak of config values

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>