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. With the 'ort' strategy, 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.

3. Other merge strategies are executed as a 'git merge-X' subcommand,
   and those strategies are currently protected with the
   'command_requires_full_index' guard.

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>
Reviewed-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 8 insertions, 0 deletions

diff --git a/merge-ort.c b/merge-ort.c
index 6eb910d6f0..1531b4c94c 100644
--- a/merge-ort.c
+++ b/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 preemptively.
+	 * 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;