builtins: annotate always-empty prefix parameters

It's usually a bad idea for a builtin's cmd_foo() to ignore the "prefix" argument it gets, as it needs to prepend that string when accessing any paths given by the user. But if a builtin does not ask for the git wrapper to run repository setup (via the RUN_SETUP or RUN_SETUP_GENTLY flags), then we know the prefix will always be NULL (it is adjusting for the chdir() done during repo setup, but there cannot be one if we did not set up the repo). In those cases it's OK to ignore "prefix", but it's worth annotating for a few reasons: 1. It serves as documentation to somebody reading the code about what we expect. 2. If the flags in git.c ever change, the run-time assertion may help detect the problem (though only if the command is run from a subdirectory of the repository). 3. It notes to the compiler that we are OK ignoring "prefix". In particular, this silences -Wunused-parameter. It _could_ also help the compiler generate better code (because it will know the prefix is NULL), but in practice this is quite unlikely to matter. Note that I've only added this annotation to commands which triggered -Wunused-parameter. It would be correct to add it to any builtin which doesn't ask for RUN_SETUP, but most of the rest of them do the sensible thing with "prefix" by passing it to parse_options(). So they're much more likely to just work if they ever switched to RUN_SETUP, and aren't worth annotating. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>