test-lib: add a SANITIZE=leak logging mode

test-lib: add a SANITIZE=leak logging mode

Add the ability to run the test suite under a new
"GIT_TEST_SANITIZE_LEAK_LOG=true" mode, when true we'll log the leaks
we find an a new "test-results/<test-name>.leak" directory.

That new path is consistent with the existing
"test-results/<test-name>.<type>" results, except that those are all
files, not directories.

We also set "log_exe_name=1" to include the name of the executable in
the filename. This gives us files like "trace.git.<pid>" instead of
the default of "trace.<pid>". I.e. we'll be able to distinguish "git"
leaks from "test-tool", "git-daemon" etc.

We then set "dedup_token_length" to non-zero ("0" is the default) to
succinctly log a token we can de-duplicate these stacktraces on. The
string is simply a one-line stack-trace with only function names up to
N frames, which we limit at "9999" as a shorthand for
"infinite" (there appears to be no way to say "no limit").

With these combined we can now easily get e.g. the top 10 leaks in the
test suite grouped by full stacktrace:

    grep -o -P -h '(?<=DEDUP_TOKEN: ).*' test-results/*.leak/trace.git.* | sort | uniq -c | sort -nr | head -n 10

Or add "grep -E -o '[^-]+'" to that to group by functions instead of
stack traces:

    grep -o -P -h '(?<=DEDUP_TOKEN: ).*' test-results/*.leak/trace.git.* | grep -E -o '[^-]+' | sort | uniq -c | sort -nr | head -n 20

This new mode requires git to be compiled with SANITIZE=leak, rather
than explaining that in the documentation let's make it
self-documenting by bailing out if the user asks for this without git
having been compiled with SANITIZE=leak, as we do with
GIT_TEST_PASSING_SANITIZE_LEAK=true.

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