read-tree: make two-way merge sparse-aware

Enable two-way merge with 'git read-tree' without expanding the sparse
index. When in a sparse index, a two-way merge will trivially succeed as
long as there are not changes to the same sparse directory in multiple trees
(i.e., sparse directory-level "edit-edit" conflicts). If there are such
conflicts, the merge will fail despite the possibility that individual files
could merge cleanly.

In order to resolve these "edit-edit" conflicts, "conflicted" sparse
directories are - rather than rejected - merged by traversing their
associated trees by OID. For each child of the sparse directory:

1. Files are merged as normal (see Documentation/git-read-tree.txt for
   details).
2. Subdirectories are treated as sparse directories and merged in
   'twoway_merge'. If there are no conflicts, they are merged according to
   the rules in Documentation/git-read-tree.txt; otherwise, the subdirectory
   is recursively traversed and merged.

This process allows sparse directories to be individually merged at the
necessary depth *without* expanding a full index.

The 't/t1092-sparse-checkout-compatibility.sh' test 'read-tree --merge with
edit/edit conflicts in sparse directories' tests two-way merges with 1)
changes inside sparse directories that do not conflict and 2) changes that
do conflict (with the correct file(s) reported in the error message).
Additionally, add two-way merge cases to 'sparse index is not expanded:
read-tree' to confirm that the index is not expanded regardless of whether
edit/edit conflicts are present in a sparse directory.

Signed-off-by: Victoria Dye <vdye@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 0 insertions, 5 deletions

diff --git a/builtin/read-tree.c b/builtin/read-tree.c
index ec6d038242..9227f07ab1 100644
--- a/builtin/read-tree.c
+++ b/builtin/read-tree.c
@@ -229,11 +229,6 @@ int cmd_read_tree(int argc, const char **argv, const char *cmd_prefix)
 			opts.fn = opts.prefix ? bind_merge : oneway_merge;
 			break;
 		case 2:
-			/*
-			 * TODO: update twoway_merge to handle edit/edit conflicts in
-			 * sparse directories.
-			 */
-			ensure_full_index(&the_index);
 			opts.fn = twoway_merge;
 			opts.initial_checkout = is_cache_unborn();
 			break;