environment: move strbuf into block to plug leak

environment: move strbuf into block to plug leak

realpath is only populated if we execute the git_work_tree_initialized
block. However that block also causes us to return early, meaning we
never actually release the strbuf in the case where we populated it.
Therefore we move all strbuf related code into the block to guarantee
that we can't leak it.

LSAN output from t0095:

Direct leak of 129 byte(s) in 1 object(s) allocated from:
    #0 0x49a9b9 in realloc ../projects/compiler-rt/lib/asan/asan_malloc_linux.cpp:164:3
    #1 0x78f585 in xrealloc wrapper.c:126:8
    #2 0x713ff4 in strbuf_grow strbuf.c:98:2
    #3 0x713ff4 in strbuf_getcwd strbuf.c:597:3
    #4 0x4f0c18 in strbuf_realpath_1 abspath.c:99:7
    #5 0x5ae4a4 in set_git_work_tree environment.c:259:3
    #6 0x6fdd8a in setup_discovered_git_dir setup.c:931:2
    #7 0x6fdd8a in setup_git_directory_gently setup.c:1235:12
    #8 0x4cb50d in get_bloom_filter_for_commit t/helper/test-bloom.c:41:2
    #9 0x4cb50d in cmd__bloom t/helper/test-bloom.c:95:3
    #10 0x4caa1f in cmd_main t/helper/test-tool.c:124:11
    #11 0x4caded in main common-main.c:52:11
    #12 0x7f0869f02349 in __libc_start_main (/lib64/libc.so.6+0x24349)

SUMMARY: AddressSanitizer: 129 byte(s) leaked in 1 allocation(s).

It looks like this leak has existed since realpath was first added to
set_git_work_tree() in:
  3d7747e318 (real_path: remove unsafe API, 2020-03-10)

Signed-off-by: Andrzej Hunt <andrzej@ahunt.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>