merge-ours: integrate with sparse-index

The merge-ours built-in opens the index to compare it against HEAD.
The machinery used to do this (i.e. run_diff_index()) is capable of
working with a sparse index, but the start-up sequence of this
command does not take the necessary steps, so we end up expanding the
index fully before doing the comparison.

In order to convince sparse-index.c:is_sparse_index_allowed() to
return true, we need to:

 - Read basic configuration with git_default_config so that global
   variables like core_apply_sparse_checkout are populated.
   merge-ours currently does not read configuration at all.

 - Set command_requires_full_index to 0.

With that, the command can work without expanding the index fully
before doing its work.

Signed-off-by: Sam Bostock <sam@sambostock.ca>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/merge-ours.c

@@ -10,6 +10,8 @@
 
 #include "git-compat-util.h"
 #include "builtin.h"
+#include "config.h"
+#include "environment.h"
 #include "diff.h"
 
 static const char builtin_merge_ours_usage[] =
@@ -22,6 +24,10 @@ int cmd_merge_ours(int argc,
 {
 	show_usage_if_asked(argc, argv, builtin_merge_ours_usage);
 
+	repo_config(repo, git_default_config, NULL);
+	prepare_repo_settings(repo);
+	repo->settings.command_requires_full_index = 0;
+
 	/*
 	 * The contents of the current index becomes the tree we
 	 * commit.  The index must match HEAD, or this merge cannot go

t/t1092-sparse-checkout-compatibility.sh

@@ -2559,4 +2559,18 @@ test_expect_success 'cat-file --batch' '
 	ensure_expanded cat-file --batch <in
 '
 
+test_expect_success 'merge -s ours' '
+	init_repos &&
+
+	test_all_match git rev-parse HEAD^{tree} &&
+	test_all_match git merge -s ours merge-right &&
+	test_all_match git rev-parse HEAD^{tree} &&
+	test_all_match git rev-parse HEAD^2
+'
+
+test_expect_success 'sparse-index is not expanded: merge-ours' '
+	init_repos &&
+	ensure_not_expanded merge -s ours merge-right
+'
+
 test_done