describe: enable sparse index for describe

git describe compares the index with the working tree when (and only
when) it is run with the "--dirty" flag. This is done by the
run_diff_index() function. The function has been made aware of the
sparse-index in the series that led to 8d2c3732 (Merge branch
'ld/sparse-diff-blame', 2021-12-21). Hence we can just set the
requires-full-index to false for "describe".

Performance metrics

  Test                                                     HEAD~1            HEAD
  -------------------------------------------------------------------------------------------------
  2000.2: git describe --dirty (full-v3)                   0.08(0.09+0.01)   0.08(0.06+0.03) +0.0%
  2000.3: git describe --dirty (full-v4)                   0.09(0.07+0.03)   0.08(0.05+0.04) -11.1%
  2000.4: git describe --dirty (sparse-v3)                 0.88(0.82+0.06)   0.02(0.01+0.05) -97.7%
  2000.5: git describe --dirty (sparse-v4)                 0.68(0.60+0.08)   0.02(0.02+0.04) -97.1%
  2000.6: echo >>new && git describe --dirty (full-v3)     0.08(0.04+0.05)   0.08(0.05+0.04) +0.0%
  2000.7: echo >>new && git describe --dirty (full-v4)     0.08(0.07+0.03)   0.08(0.05+0.04) +0.0%
  2000.8: echo >>new && git describe --dirty (sparse-v3)   0.75(0.69+0.07)   0.02(0.03+0.03) -97.3%
  2000.9: echo >>new && git describe --dirty (sparse-v4)   0.81(0.73+0.09)   0.02(0.01+0.05) -97.5%

Signed-off-by: Raghul Nanth A <nanth.raghul@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 25 insertions, 0 deletions

diff --git a/t/t1092-sparse-checkout-compatibility.sh b/t/t1092-sparse-checkout-compatibility.sh
index 801919009e..51bebd4fa9 100755
--- a/t/t1092-sparse-checkout-compatibility.sh
+++ b/t/t1092-sparse-checkout-compatibility.sh
@@ -1514,6 +1514,31 @@ test_expect_success 'sparse-index is not expanded: stash' '
 	ensure_not_expanded stash pop
 '
 
+test_expect_success 'describe tested on all' '
+	init_repos &&
+
+	# Add tag to be read by describe
+
+	run_on_all git tag -a v1.0 -m "Version 1" &&
+	test_all_match git describe --dirty &&
+	run_on_all rm g &&
+	test_all_match git describe --dirty
+'
+
+
+test_expect_success 'sparse-index is not expanded: describe' '
+	init_repos &&
+
+	# Add tag to be read by describe
+
+	git -C sparse-index tag -a v1.0 -m "Version 1" &&
+
+	ensure_not_expanded describe --dirty &&
+	echo "test" >>sparse-index/g &&
+	ensure_not_expanded describe --dirty &&
+	ensure_not_expanded describe
+'
+
 test_expect_success 'sparse index is not expanded: diff' '
 	init_repos &&