[PATCH] contrib/subtree: fix "subtree split" skipped-merge bug.

[Dave Ware <davidw@realtimegenomics.com>]

A bug occurs in 'git-subtree split' where a merge is skipped even when
both parents act on the subtree, provided the merge results in a tree
identical to one of the parents. Fix by copying the merge if at least
one parent is non-identical, and the non-identical parent is not an
ancestor of the identical parent.

Also adding a test case, this checks that a descendant can be pushed to
it's ancestor in this case.

Signed-off-by: Dave Ware <davidw@realtimegenomics.com>
---
 contrib/subtree/git-subtree.sh     | 12 +++++++--
 contrib/subtree/t/t7900-subtree.sh | 52 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 62 insertions(+), 2 deletions(-)

diff --git a/contrib/subtree/git-subtree.sh b/contrib/subtree/git-subtree.sh
index 9f06571..b837531 100755
--- a/contrib/subtree/git-subtree.sh
+++ b/contrib/subtree/git-subtree.sh
@@ -479,8 +479,16 @@ copy_or_skip()
 			p="$p -p $parent"
 		fi
 	done
-	
-	if [ -n "$identical" ]; then
+
+	copycommit=
+	if [ -n "$identical" ] && [ -n "$nonidentical" ]; then
+		extras=$(git rev-list --boundary $identical..$nonidentical)
+		if [ -n "$extras" ]; then
+			# we need to preserve history along the other branch
+			copycommit=1
+		fi
+	fi
+	if [ -n "$identical" ] && [ -z "$copycommit" ]; then
 		echo $identical
 	else
 		copy_commit $rev $tree "$p" || exit $?
diff --git a/contrib/subtree/t/t7900-subtree.sh b/contrib/subtree/t/t7900-subtree.sh
index 9051982..ea991eb 100755
--- a/contrib/subtree/t/t7900-subtree.sh
+++ b/contrib/subtree/t/t7900-subtree.sh
@@ -468,4 +468,56 @@ test_expect_success 'verify one file change per commit' '
 	))
 '
 
+test_expect_success 'subtree descendent check' '
+  mkdir git_subtree_split_check &&
+  (
+    cd git_subtree_split_check &&
+    git init &&
+
+    mkdir folder &&
+
+    echo a >folder/a &&
+    git add . &&
+    git commit -m "first commit" &&
+
+    git branch branch &&
+
+    echo 0 >folder/0 &&
+    git add . &&
+    git commit -m "adding 0 to folder" &&
+
+    echo b >folder/b &&
+    git add . &&
+    git commit -m "adding b to folder" &&
+    cherry=$(git rev-list HEAD -1) &&
+
+    git checkout branch &&
+    echo text >textBranch.txt &&
+    git add . &&
+    git commit -m "commit to fiddle with branch: branch" &&
+
+    git cherry-pick $cherry &&
+    git checkout master &&
+    git merge -m "merge" branch &&
+
+    git branch noop_branch &&
+
+    echo d >folder/d &&
+    git add . &&
+    git commit -m "adding d to folder" &&
+
+    git checkout noop_branch &&
+    echo moreText >anotherText.txt &&
+    git add . &&
+    git commit -m "irrelevant" &&
+
+    git checkout master &&
+    git merge -m "second merge" noop_branch &&
+
+    git subtree split --prefix folder/ --branch subtree_tip master &&
+    git subtree split --prefix folder/ --branch subtree_branch branch &&
+    git push . subtree_tip:subtree_branch
+  )
+  '
+
 test_done
-- 
1.9.1