blame: enable and test the sparse index

Enable the sparse index for the 'git blame' command. The index was already
not expanded with this command, so the most interesting thing to do is
to add tests that verify that 'git blame' behaves
correctly when the sparse index is enabled and that its performance
improves. More specifically, these cases are...

1. The index is not expanded for `blame` when given paths in the sparse
checkout cone at multiple levels.
2. Performance measurably improves for `blame` with sparse index when
given paths in the sparse checkout cone at multiple levels.

The following are the performance results comparing the branch with
sparse index enabled for `blame` with the vfs-2.33.0 branch in the
microsoft/git repository:

```
Test                                       msft/vfs-2.33.0   sparse-index-blame
-----------------------------------------------------------------------------------
2000.2: git blame f2/f4/a (full-v3)        0.31(0.21+0.09)   0.32(0.21+0.10) +3.2%
2000.3: git blame f2/f4/a (full-v4)        0.29(0.19+0.08)   0.31(0.21+0.08) +6.9%
2000.4: git blame f2/f4/a (sparse-v3)      0.55(0.38+0.14)   0.23(0.14+0.07) -58.2%
2000.5: git blame f2/f4/a (sparse-v4)      0.57(0.40+0.16)   0.23(0.15+0.07) -59.6%
2000.6: git blame f2/f4/f3/a (full-v3)     0.77(0.56+0.20)   0.85(0.60+0.22) +10.4%
2000.7: git blame f2/f4/f3/a (full-v4)     0.78(0.56+0.20)   0.81(0.59+0.21) +3.8%
2000.8: git blame f2/f4/f3/a (sparse-v3)   1.07(0.78+0.28)   0.72(0.52+0.19) -32.7%
2000.9: git blame f2/f4/f3/a (sparse-v4)   1.05(0.78+0.26)   0.73(0.51+0.19) -30.5%
```

We do not include paths outside the sparse checkout cone because blame
currently does not support blaming files outside of the sparse
definition.

builtin/blame.c

@@ -902,6 +902,8 @@ int cmd_blame(int argc, const char **argv, const char *prefix)
 	long anchor;
 	const int hexsz = the_hash_algo->hexsz;
 
+	prepare_repo_settings(the_repository);
+	the_repository->settings.command_requires_full_index = 0;
 	setup_default_color_by_age();
 	git_config(git_blame_config, &output_option);
 	repo_init_revisions(the_repository, &revs, NULL);

t/perf/p2000-sparse-operations.sh

@@ -120,5 +120,7 @@ test_perf_on_all git update-index --add --remove
 test_perf_on_all git diff
 test_perf_on_all git diff --staged
 test_perf_on_all git sparse-checkout reapply
+test_perf_on_all git blame f2/f4/a
+test_perf_on_all git blame f2/f4/f3/a
 
 test_done

t/t1092-sparse-checkout-compatibility.sh

@@ -1280,6 +1280,15 @@ test_expect_success 'sparse index is not expanded: read-tree' '
 	ensure_not_expanded read-tree --prefix=deep/deeper2 -u deepest
 '
 
+test_expect_success 'sparse index is not expanded: blame' '
+	init_repos &&
+
+	ensure_not_expanded blame a &&
+	ensure_not_expanded blame deep/a &&
+	ensure_not_expanded blame deep/deeper1/a &&
+	ensure_not_expanded blame deep/deeper1/deepest/a
+'
+
 # NEEDSWORK: a sparse-checkout behaves differently from a full checkout
 # in this scenario, but it shouldn't.
 test_expect_success 'reset mixed and checkout orphan' '