Fix dropping submodule changes from a commit

Our logic to decide if a file needs to be checked out from the previous commit
or deleted because it didn't exist in the previous commit didn't work for
submodules. We were using `git cat-file -e` to ask whether the file existed, but
this returns an error for submodules, so we were always deleting those instead
of reverting them back to their previous state.

Switch to using `git ls-tree -- file` instead, which works for both files and
submodules.

pkg/commands/git_commands/rebase.go

@@ -521,10 +521,17 @@ func (self *RebaseCommands) DiscardOldFileChanges(commits []*models.Commit, comm
 	}
 
 	for _, filePath := range filePaths {
-		// check if file exists in previous commit (this command returns an error if the file doesn't exist)
+		doesFileExistInPreviousCommit := false
-		cmdArgs := NewGitCmd("cat-file").Arg("-e", "HEAD^:"+filePath).ToArgv()
+		if commitIndex < len(commits)-1 {
-
+			// check if file exists in previous commit (this command returns an empty string if the file doesn't exist)
-		if err := self.cmd.New(cmdArgs).Run(); err != nil {
+			cmdArgs := NewGitCmd("ls-tree").Arg("--name-only", "HEAD^", "--", filePath).ToArgv()
+			output, err := self.cmd.New(cmdArgs).DontLog().RunWithOutput()
+			if err != nil {
+				return err
+			}
+			doesFileExistInPreviousCommit = strings.TrimRight(output, "\n") == filePath
+		}
+		if !doesFileExistInPreviousCommit {
 			if err := self.os.Remove(filePath); err != nil {
 				return err
 			}

pkg/commands/git_commands/rebase_test.go

@@ -139,7 +139,7 @@ func TestRebaseDiscardOldFileChanges(t *testing.T) {
 			fileName:    []string{"test999.txt"},
 			runner: oscommands.NewFakeRunner(t).
 				ExpectGitArgs([]string{"rebase", "--interactive", "--autostash", "--keep-empty", "--no-autosquash", "--rebase-merges", "abcdef"}, "", nil).
-				ExpectGitArgs([]string{"cat-file", "-e", "HEAD^:test999.txt"}, "", nil).
+				ExpectGitArgs([]string{"ls-tree", "--name-only", "HEAD^", "--", "test999.txt"}, "test999.txt\n", nil).
 				ExpectGitArgs([]string{"checkout", "HEAD^", "--", "test999.txt"}, "", nil).
 				ExpectGitArgs([]string{"commit", "--amend", "--no-edit", "--allow-empty", "--allow-empty-message"}, "", nil).
 				ExpectGitArgs([]string{"rebase", "--continue"}, "", nil),

pkg/integration/tests/commit/discard_submodule_changes.go

@@ -49,9 +49,6 @@ var DiscardSubmoduleChanges = NewIntegrationTest(NewIntegrationTestArgs{
 			Confirm()
 
 		t.Shell().RunCommand([]string{"git", "submodule", "update"})
-		/* EXPECTED:
 		t.FileSystem().FileContent("submodule/file", Equals("content"))
-		ACTUAL: */
-		t.FileSystem().PathNotPresent("submodule/file")
 	},
 })