git-p4: Verify detection of "empty" branch creation

Current implementation of new branch parent detection works on the
principle that the new branch is a complete integration, with no
changes, of the original files.
This test shows this deficiency in the particular case when the new
branch is created from a subset of the original files.

Signed-off-by: Vitor Antunes <vitor.hda@gmail.com>
Acked-by: Pete Wyckoff <pw@padd.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t9801-git-p4-branch.sh

@@ -358,6 +358,60 @@ test_expect_success 'git p4 sync changes to two branches in the same changelist'
 	)
 '
 
+# Create a branch by integrating a single file
+test_expect_success 'git p4 file subset branch' '
+	(
+		cd "$cli" &&
+		p4 integrate //depot/branch1/file1 //depot/branch6/file1 &&
+		p4 submit -d "Integrate file1 alone from branch1 to branch6"
+	)
+'
+
+# Check if git p4 creates a new branch containing a single file,
+# instead of keeping the old files from the original branch
+test_expect_failure 'git p4 clone file subset branch' '
+	test_when_finished cleanup_git &&
+	test_create_repo "$git" &&
+	(
+		cd "$git" &&
+		git config git-p4.branchList branch1:branch2 &&
+		git config --add git-p4.branchList branch1:branch3 &&
+		git config --add git-p4.branchList branch1:branch4 &&
+		git config --add git-p4.branchList branch1:branch5 &&
+		git config --add git-p4.branchList branch1:branch6 &&
+		git p4 clone --dest=. --detect-branches //depot@all &&
+		git log --all --graph --decorate --stat &&
+		git reset --hard p4/depot/branch1 &&
+		test_path_is_file file1 &&
+		test_path_is_file file2 &&
+		test_path_is_file file3 &&
+		grep update file2 &&
+		git reset --hard p4/depot/branch2 &&
+		test_path_is_file file1 &&
+		test_path_is_file file2 &&
+		test_path_is_missing file3 &&
+		! grep update file2 &&
+		git reset --hard p4/depot/branch3 &&
+		test_path_is_file file1 &&
+		test_path_is_file file2 &&
+		test_path_is_missing file3 &&
+		grep update file2 &&
+		git reset --hard p4/depot/branch4 &&
+		test_path_is_file file1 &&
+		test_path_is_file file2 &&
+		test_path_is_file file3 &&
+		! grep update file2 &&
+		git reset --hard p4/depot/branch5 &&
+		test_path_is_file file1 &&
+		test_path_is_file file2 &&
+		test_path_is_file file3 &&
+		! grep update file2 &&
+		git reset --hard p4/depot/branch6 &&
+		test_path_is_file file1 &&
+		test_path_is_missing file2 &&
+		test_path_is_missing file3
+	)
+'
 test_expect_success 'kill p4d' '
 	kill_p4d
 '