contrib/subtree: process out-of-prefix subtrees

`should_ignore_subtree_split_commit` detects subtrees which are
outside of the current path --prefix and ignores them. This can
speed up splits of repositories that have many subtrees.

Since its inception [1], every iteration of this logic [2], [3]
incorrectly excludes commits. This alters the split history. The
split history and its commit hashes are API contract, so this is
not permissible.

While a commit from a different subtree may look like it doesn't
contribute anything to a split, sometimes it does. Merge commits
are a particular hot spot. For these, the pruning logic in
`copy_or_skip` performs:

1. a check for "treesame" parents
2. two different common ancestry checks

These checks operate on the **split history**, not the input
history. The split history omits commits that do not affect the
--prefix. This can significantly alter the ancestry of a merge.
In order to determine if `copy_or_skip` will skip a merge, it
is likely necessary to compute all the split history... which
is what `should_ignore_subtree_split_commit` tries to avoid.

To make this logic API-preserving, we could gate it behind a
new CLI argument. The present implementation is actually a
speed penalty in many cases, however, so this is not done here.

Remove the `should_ignore_subtree_split_commit` logic. This
fixes the regression reported in [4].

[1]: 98ba49ccc2 (subtree: fix split processing with multiple subtrees present, 2023-12-01)

[2]: 83f9dad7d6 (contrib/subtree: fix split with squashed subtrees, 2025-09-09)

[3]: 28a7e27cff (contrib/subtree: detect rewritten subtree commits, 2026-01-09)

[4]: <20251230170719.845029-1-george@mail.dietrich.pub>

Reported-by: George <george@mail.dietrich.pub>
Reported-by: Christian Heusel <christian@heusel.eu>
Signed-off-by: Colin Stagner <ask+git@howdoi.land>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 1 insertions, 49 deletions

diff --git a/contrib/subtree/git-subtree.sh b/contrib/subtree/git-subtree.sh
index 17106d1a72..ba9fb2ee5d 100755
--- a/contrib/subtree/git-subtree.sh
+++ b/contrib/subtree/git-subtree.sh
@@ -785,42 +785,6 @@ ensure_valid_ref_format () {
 		die "fatal: '$1' does not look like a ref"
 }
 
-# Usage: should_ignore_subtree_split_commit REV
-#
-# Check if REV is a commit from another subtree and should be
-# ignored from processing for splits
-should_ignore_subtree_split_commit () {
-	assert test $# = 1
-
-	git show \
-		--no-patch \
-		--no-show-signature \
-		--format='%(trailers:key=git-subtree-dir,key=git-subtree-mainline)' \
-		"$1" |
-	(
-	have_mainline=
-	subtree_dir=
-
-	while read -r trailer val
-	do
-		case "$trailer" in
-		git-subtree-dir:)
-			subtree_dir="${val%/}" ;;
-		git-subtree-mainline:)
-			have_mainline=y ;;
-		esac
-	done
-
-	if test -n "${subtree_dir}" &&
-		test -z "${have_mainline}" &&
-		test "${subtree_dir}" != "$arg_prefix"
-	then
-		return 0
-	fi
-	return 1
-	)
-}
-
 # Usage: process_split_commit REV PARENTS
 process_split_commit () {
 	assert test $# = 2
@@ -1006,19 +970,7 @@ cmd_split () {
 	eval "$grl" |
 	while read rev parents
 	do
-		if should_ignore_subtree_split_commit "$rev"
-		then
-			continue
-		fi
-		parsedparents=''
-		for parent in $parents
-		do
-			if ! should_ignore_subtree_split_commit "$parent"
-			then
-				parsedparents="$parsedparents$parent "
-			fi
-		done
-		process_split_commit "$rev" "$parsedparents"
+		process_split_commit "$rev" "$parents"
 	done || exit $?
 
 	latest_new=$(cache_get latest_new) || exit $?