t/perf: add basic perf tests for delta base cache

This just shows off the improvements done by the last few
patches, and gives us a baseline for noticing regressions in
the future. Here are the results with linux.git as the perf
"large repo":

Test                origin                HEAD
-------------------------------------------------------------------
0003.1: log --raw   43.41(40.36+2.69)     33.86(30.96+2.41) -22.0%
0003.2: log -S      313.61(309.74+3.78)   298.75(295.58+3.00) -4.7%

(for a large repo, the "log -S" improvements are greater if
you bump the delta base cache limit, but I think it makes
sense to test the "stock" behavior, since that is what most
people will see).

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

t/perf/p0003-delta-base-cache.sh

@@ -0,0 +1,31 @@
+#!/bin/sh
+
+test_description='Test operations that emphasize the delta base cache.
+
+We look at both "log --raw", which should put only trees into the delta cache,
+and "log -Sfoo --raw", which should look at both trees and blobs.
+
+Any effects will be emphasized if the test repository is fully packed (loose
+objects obviously do not use the delta base cache at all). It is also
+emphasized if the pack has long delta chains (e.g., as produced by "gc
+--aggressive"), though cache is still quite noticeable even with the default
+depth of 50.
+
+The setting of core.deltaBaseCacheLimit in the source repository is also
+relevant (depending on the size of your test repo), so be sure it is consistent
+between runs.
+'
+. ./perf-lib.sh
+
+test_perf_large_repo
+
+# puts mostly trees into the delta base cache
+test_perf 'log --raw' '
+	git log --raw >/dev/null
+'
+
+test_perf 'log -S' '
+	git log --raw -Sfoo >/dev/null
+'
+
+test_done