bloom: clear each bloom_key after use

bloom: clear each bloom_key after use

fill_bloom_key() allocates memory into bloom_key, we need to clean that
up once the key is no longer needed.

This leak was found while running t0002-t0099. Although this leak is
happening in code being called from a test-helper, the same code is also
used in various locations around git, and can therefore happen during
normal usage too. Gabor's analysis shows that peak-memory usage during
'git commit-graph write' is reduced on the order of 10% for a selection
of larger repos (along with an even larger reduction if we override
modified path bloom filter limits):
https://lore.kernel.org/git/20210411072651.GF2947267@szeder.dev/

LSAN output:

Direct leak of 308 byte(s) in 11 object(s) allocated from:
    #0 0x49a5e2 in calloc ../projects/compiler-rt/lib/asan/asan_malloc_linux.cpp:154:3
    #1 0x6f4032 in xcalloc wrapper.c:140:8
    #2 0x4f2905 in fill_bloom_key bloom.c:137:28
    #3 0x4f34c1 in get_or_compute_bloom_filter bloom.c:284:4
    #4 0x4cb484 in get_bloom_filter_for_commit t/helper/test-bloom.c:43:11
    #5 0x4cb072 in cmd__bloom t/helper/test-bloom.c:97:3
    #6 0x4ca7ef in cmd_main t/helper/test-tool.c:121:11
    #7 0x4caace in main common-main.c:52:11
    #8 0x7f798af95349 in __libc_start_main (/lib64/libc.so.6+0x24349)

SUMMARY: AddressSanitizer: 308 byte(s) leaked in 11 allocation(s).

Signed-off-by: Andrzej Hunt <ajrhunt@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>