test-lib: set ASAN_OPTIONS variable before we run git

We turn off ASan's leak detection by default in the test
suite because it's too noisy. But we don't do so until
part-way through test-lib. This is before we've run any
tests, but after we do our initial "./git" to see if the
binary has even been built.

When built with clang, this seems to work fine. However,
using "gcc -fsanitize=address", the leak checker seems to
complain more aggressively:

  $ ./git
  ...
  ==5352==ERROR: LeakSanitizer: detected memory leaks
  Direct leak of 2 byte(s) in 1 object(s) allocated from:
      #0 0x7f120e7afcf8 in malloc (/usr/lib/x86_64-linux-gnu/libasan.so.3+0xc1cf8)
      #1 0x559fc2a3ce41 in do_xmalloc /home/peff/compile/git/wrapper.c:60
      #2 0x559fc2a3cf1a in do_xmallocz /home/peff/compile/git/wrapper.c:100
      #3 0x559fc2a3d0ad in xmallocz /home/peff/compile/git/wrapper.c:108
      #4 0x559fc2a3d0ad in xmemdupz /home/peff/compile/git/wrapper.c:124
      #5 0x559fc2a3d0ad in xstrndup /home/peff/compile/git/wrapper.c:130
      #6 0x559fc274535a in main /home/peff/compile/git/common-main.c:39
      #7 0x7f120dabd2b0 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x202b0)

This is a leak in the sense that we never free it, but it's
in a global that is meant to last the whole program. So it's
not really interesting or in need of fixing. And at any
rate, mentioning leaks outside of the test_expect blocks is
certainly unwelcome, as it pollutes stderr.

Let's bump the setting of ASAN_OPTIONS higher in test-lib.sh
to catch our initial "can we even run git?" test.  While
we're at it, we can add a comment to make it a bit less
inscrutable.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/test-lib.sh

@@ -36,6 +36,14 @@ then
 fi
 GIT_BUILD_DIR="$TEST_DIRECTORY"/..
 
+# If we were built with ASAN, it may complain about leaks
+# of program-lifetime variables. Disable it by default to lower
+# the noise level. This needs to happen at the start of the script,
+# before we even do our "did we build git yet" check (since we don't
+# want that one to complain to stderr).
+: ${ASAN_OPTIONS=detect_leaks=0}
+export ASAN_OPTIONS
+
 ################################################################
 # It appears that people try to run tests without building...
 "$GIT_BUILD_DIR/git" >/dev/null
@@ -148,9 +156,6 @@ else
 	}
 fi
 
-: ${ASAN_OPTIONS=detect_leaks=0}
-export ASAN_OPTIONS
-
 # Protect ourselves from common misconfiguration to export
 # CDPATH into the environment
 unset CDPATH