t/perf: export variable used in other blocks

In p0001, a variable was created in a test_expect_success block to be
used in later test_perf blocks, but was not exported. This caused the
variable to not appear in those blocks (this can be verified by writing
'test -n "$commit"' in those blocks), resulting in a slightly different
invocation than what was intended. Export that variable.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/perf/p0001-rev-list.sh

@@ -15,7 +15,8 @@ test_perf 'rev-list --all --objects' '
 '
 
 test_expect_success 'create new unreferenced commit' '
-	commit=$(git commit-tree HEAD^{tree} -p HEAD)
+	commit=$(git commit-tree HEAD^{tree} -p HEAD) &&
+	test_export commit
 '
 
 test_perf 'rev-list $commit --not --all' '