merge: make sparse-aware with ORT

Allow 'git merge' to operate without expanding a sparse index, at least
not immediately. The index still will be expanded in a few cases:

1. If the merge strategy is 'recursive', then we enable
   command_requires_full_index at the start of the merge_recursive()
   method. We expect sparse-index users to also have the 'ort' strategy
   enabled.

2. If the merge results in a conflicted file, then we expand the index
   before updating the working tree. The loop that iterates over the
   worktree replaces index entries and tracks 'origintal_cache_nr' which
   can become completely wrong if the index expands in the middle of the
   operation. This safety valve is important before that loop starts. A
   later change will focus this to only expand if we indeed have a
   conflict outside of the sparse-checkout cone.

Some test updates are required, including a mistaken 'git checkout -b'
that did not specify the base branch, causing merges to be fast-forward
merges.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>

builtin/merge.c

@@ -1276,6 +1276,9 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 	if (argc == 2 && !strcmp(argv[1], "-h"))
 		usage_with_options(builtin_merge_usage, builtin_merge_options);
 
+	prepare_repo_settings(the_repository);
+	the_repository->settings.command_requires_full_index = 0;
+
 	/*
 	 * Check if we are _not_ on a detached HEAD, i.e. if there is a
 	 * current branch.

merge-ort.c

@@ -4058,6 +4058,14 @@ static int record_conflicted_index_entries(struct merge_options *opt)
 	if (strmap_empty(&opt->priv->conflicted))
 		return 0;
 
+	/*
+	 * We are in a conflicted state. These conflicts might be inside
+	 * sparse-directory entries, so expand the index preemtively.
+	 * Also, we set original_cache_nr below, but that might change if
+	 * index_name_pos() calls ask for paths within sparse directories.
+	 */
+	ensure_full_index(index);
+
 	/* If any entries have skip_worktree set, we'll have to check 'em out */
 	state.force = 1;
 	state.quiet = 1;

merge-recursive.c

@@ -3750,6 +3750,9 @@ int merge_recursive(struct merge_options *opt,
 	assert(opt->ancestor == NULL ||
 	       !strcmp(opt->ancestor, "constructed merge base"));
 
+	prepare_repo_settings(opt->repo);
+	opt->repo->settings.command_requires_full_index = 1;
+
 	if (merge_start(opt, repo_get_commit_tree(opt->repo, h1)))
 		return -1;
 	clean = merge_recursive_internal(opt, h1, h2, merge_bases, result);

t/t1092-sparse-checkout-compatibility.sh

@@ -47,7 +47,7 @@ test_expect_success 'setup' '
 		git checkout -b base &&
 		for dir in folder1 folder2 deep
 		do
-			git checkout -b update-$dir &&
+			git checkout -b update-$dir base &&
 			echo "updated $dir" >$dir/a &&
 			git commit -a -m "update $dir" || return 1
 		done &&
@@ -647,7 +647,15 @@ test_expect_success 'sparse-index is not expanded' '
 	echo >>sparse-index/extra.txt &&
 	ensure_not_expanded add extra.txt &&
 	echo >>sparse-index/untracked.txt &&
-	ensure_not_expanded add .
+	ensure_not_expanded add . &&
+
+	ensure_not_expanded checkout -f update-deep &&
+	(
+		sane_unset GIT_TEST_MERGE_ALGORITHM &&
+		git -C sparse-index config pull.twohead ort &&
+		ensure_not_expanded merge -m merge update-folder1 &&
+		ensure_not_expanded merge -m merge update-folder2
+	)
 '
 
 # NEEDSWORK: a sparse-checkout behaves differently from a full checkout