test-lib: make $GIT_BUILD_DIR an absolute path

Change the GIT_BUILD_DIR from a path like "/path/to/build/t/.." to
"/path/to/build". The "TEST_DIRECTORY" here is already made an
absolute path a few lines above this.

We could simply do $(cd "$TEST_DIRECTORY"/.." && pwd) here, but as
noted in the preceding commit the "$TEST_DIRECTORY" can't be anything
except the path containing this test-lib.sh file at this point, so we
can more cheaply and equally strip the "/t" off the end.

This change will be helpful to LSAN_OPTIONS which will want to strip
the build directory path from filenames, which we couldn't do if we
had a "/.." in there.

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

t/test-lib.sh

@@ -41,8 +41,8 @@ then
 	# elsewhere
 	TEST_OUTPUT_DIRECTORY=$TEST_DIRECTORY
 fi
-GIT_BUILD_DIR="$TEST_DIRECTORY"/..
-if test "$TEST_DIRECTORY" = "${TEST_DIRECTORY%/t}"
+GIT_BUILD_DIR="${TEST_DIRECTORY%/t}"
+if test "$TEST_DIRECTORY" = "$GIT_BUILD_DIR"
 then
 	echo "PANIC: Running in a $TEST_DIRECTORY that doesn't end in '/t'?" >&2
 	exit 1
@@ -64,6 +64,7 @@ prepend_var () {
 # problems. The GIT_SAN_OPTIONS variable can be used to set common
 # defaults shared between [AL]SAN_OPTIONS.
 prepend_var GIT_SAN_OPTIONS : abort_on_error=1
+prepend_var GIT_SAN_OPTIONS : strip_path_prefix=\"$GIT_BUILD_DIR/\"
 
 # If we were built with ASAN, it may complain about leaks
 # of program-lifetime variables. Disable it by default to lower