test-lib: add "fast_unwind_on_malloc=0" to LSAN_OPTIONS

Add "fast_unwind_on_malloc=0" to LSAN_OPTIONS to get more meaningful
stack traces from LSAN. This isn't required under ASAN which will emit
traces such as this one for a leak in "t/t0006-date.sh":

    $ ASAN_OPTIONS=detect_leaks=1 ./t0006-date.sh -vixd
    [...]
    Direct leak of 3 byte(s) in 1 object(s) allocated from:
        #0 0x488b94 in strdup (t/helper/test-tool+0x488b94)
        #1 0x9444a4 in xstrdup wrapper.c:29:14
        microsoft#2 0x5995fa in parse_date_format date.c:991:24
        microsoft#3 0x4d2056 in show_dates t/helper/test-date.c:39:2
        microsoft#4 0x4d174a in cmd__date t/helper/test-date.c:116:3
        microsoft#5 0x4cce89 in cmd_main t/helper/test-tool.c:127:11
        microsoft#6 0x4cd1e3 in main common-main.c:52:11
        microsoft#7 0x7fef3c695e49 in __libc_start_main csu/../csu/libc-start.c:314:16
        microsoft#8 0x422b09 in _start (t/helper/test-tool+0x422b09)

    SUMMARY: AddressSanitizer: 3 byte(s) leaked in 1 allocation(s).
    Aborted

Whereas LSAN would emit this instead:

    $ ./t0006-date.sh -vixd
    [...]
    Direct leak of 3 byte(s) in 1 object(s) allocated from:
        #0 0x4323b8 in malloc (t/helper/test-tool+0x4323b8)
        #1 0x7f2be1d614aa in strdup string/strdup.c:42:15

    SUMMARY: LeakSanitizer: 3 byte(s) leaked in 1 allocation(s).
    Aborted

Now we'll instead git this sensible stack trace under
LSAN. I.e. almost the same one (but starting with "malloc", as is
usual for LSAN) as under ASAN:

    Direct leak of 3 byte(s) in 1 object(s) allocated from:
        #0 0x4323b8 in malloc (t/helper/test-tool+0x4323b8)
        #1 0x7f012af5c4aa in strdup string/strdup.c:42:15
        microsoft#2 0x5cb164 in xstrdup wrapper.c:29:14
        microsoft#3 0x495ee9 in parse_date_format date.c:991:24
        microsoft#4 0x453aac in show_dates t/helper/test-date.c:39:2
        microsoft#5 0x453782 in cmd__date t/helper/test-date.c:116:3
        microsoft#6 0x451d95 in cmd_main t/helper/test-tool.c:127:11
        microsoft#7 0x451f1e in main common-main.c:52:11
        microsoft#8 0x7f012aef5e49 in __libc_start_main csu/../csu/libc-start.c:314:16
        microsoft#9 0x42e0a9 in _start (t/helper/test-tool+0x42e0a9)

    SUMMARY: LeakSanitizer: 3 byte(s) leaked in 1 allocation(s).
    Aborted

As the option name suggests this does make things slower, e.g. for
t0001-init.sh we're around 10% slower:

    $ hyperfine -L v 0,1 'LSAN_OPTIONS=fast_unwind_on_malloc={v} make T=t0001-init.sh' -r 3
    Benchmark 1: LSAN_OPTIONS=fast_unwind_on_malloc=0 make T=t0001-init.sh
      Time (mean ± σ):      2.135 s ±  0.015 s    [User: 1.951 s, System: 0.554 s]
      Range (min … max):    2.122 s …  2.152 s    3 runs

    Benchmark 2: LSAN_OPTIONS=fast_unwind_on_malloc=1 make T=t0001-init.sh
      Time (mean ± σ):      1.981 s ±  0.055 s    [User: 1.769 s, System: 0.488 s]
      Range (min … max):    1.941 s …  2.044 s    3 runs

    Summary
      'LSAN_OPTIONS=fast_unwind_on_malloc=1 make T=t0001-init.sh' ran
        1.08 ± 0.03 times faster than 'LSAN_OPTIONS=fast_unwind_on_malloc=0 make T=t0001-init.sh'

I think that's more than worth it to get the more meaningful stack
traces, we can always provide LSAN_OPTIONS=fast_unwind_on_malloc=0 for
one-off "fast" runs.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/test-lib.sh

@@ -76,6 +76,7 @@ prepend_var ASAN_OPTIONS : detect_leaks=0
 export ASAN_OPTIONS
 
 prepend_var LSAN_OPTIONS : $GIT_SAN_OPTIONS
+prepend_var LSAN_OPTIONS : fast_unwind_on_malloc=0
 export LSAN_OPTIONS
 
 if test ! -f "$GIT_BUILD_DIR"/GIT-BUILD-OPTIONS