t/perf: time rev-list with UNINTERESTING commits

We time a straight "rev-list --all" and its "--object"
counterpart, both going all the way to the root. However, we
do not time a partial history walk. This patch adds an
extreme case: a walk over a very small slice of history, but
with a very large set of UNINTERESTING tips. This is similar
to the connectivity check run by git on a small fetch, or
the walk done by any pre-receive hooks that want to check
incoming commits.

This test reveals a performance regression in git v1.8.4.2,
caused by fbd4a70 (list-objects: mark more commits as edges
in mark_edges_uninteresting, 2013-08-16):

Test                                             fbd4a70^         fbd4a70
------------------------------------------------------------------------------------------
0001.1: rev-list --all                           0.69(0.67+0.02)   0.69(0.68+0.01) +0.0%
0001.2: rev-list --all --objects                 3.47(3.44+0.02)   3.48(3.44+0.03) +0.3%
0001.4: rev-list $commit --not --all             0.04(0.04+0.00)   0.04(0.04+0.00) +0.0%
0001.5: rev-list --objects $commit --not --all   0.04(0.03+0.00)   0.27(0.24+0.02) +575.0%

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

t/perf/p0001-rev-list.sh

@@ -14,4 +14,16 @@ test_perf 'rev-list --all --objects' '
 	git rev-list --all --objects >/dev/null
 '
 
+test_expect_success 'create new unreferenced commit' '
+	commit=$(git commit-tree HEAD^{tree} -p HEAD)
+'
+
+test_perf 'rev-list $commit --not --all' '
+	git rev-list $commit --not --all >/dev/null
+'
+
+test_perf 'rev-list --objects $commit --not --all' '
+	git rev-list --objects $commit --not --all >/dev/null
+'
+
 test_done