merge-ours: integrate with sparse-index

The merge-ours builtin reads the index only to compare it against HEAD
via index_differs_from(), whose diff machinery (run_diff_index) is
already sparse-aware.

Teach merge-ours to opt out of requiring a full index by setting
command_requires_full_index to 0. Because merge-ours is invoked as a
subprocess by "git merge -s ours" and never previously read config,
the global variables core_apply_sparse_checkout and
core_sparse_checkout_cone remained unset, causing
is_sparse_index_allowed() to return false and the index to be expanded
anyway. Add a repo_config() call with git_default_config to populate
these globals.

Add tests to t1092 verifying that "git merge -s ours" produces
identical results across full-checkout, sparse-checkout, and
sparse-index modes, including verifying the resulting merge commit
structure, and that the sparse index is not expanded during the
operation.

Signed-off-by: Sam Bostock <sam@bostock.ca>

Author: sambostock

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