Tolerate trailing newlines in AddCoAuthorToDescription

Callers currently hand this function a trimmed description, so the output is
always clean. An upcoming change to the commit-panel getters will stop trimming
at the callsite (so that whitespace typed by the user round-trips through the
preservation file exactly), at which point the description can end with one or
more newlines. Without this change, a user who presses Enter after their
description body and then invokes "Add co-author" would end up with two blank
lines between the body and the trailer instead of the expected one.

pkg/commands/git_commands/commit.go

@@ -61,6 +61,7 @@ func AddCoAuthorToMessage(message string, author string) string {
 }
 
 func AddCoAuthorToDescription(description string, author string) string {
+	description = strings.TrimRight(description, "\n")
 	if description != "" {
 		lines := strings.Split(description, "\n")
 		if strings.HasPrefix(lines[len(lines)-1], "Co-authored-by:") {

pkg/commands/git_commands/commit_test.go

@@ -483,6 +483,11 @@ func TestAddCoAuthorToDescription(t *testing.T) {
 			description:    "Body\n\nCo-authored-by: Jane Smith <jane@smith.com>",
 			expectedResult: "Body\n\nCo-authored-by: Jane Smith <jane@smith.com>\nCo-authored-by: John Doe <john@doe.com>",
 		},
+		{
+			name:           "Description with trailing newlines",
+			description:    "Body\n\n",
+			expectedResult: "Body\n\nCo-authored-by: John Doe <john@doe.com>",
+		},
 	}
 	for _, s := range scenarios {
 		t.Run(s.name, func(t *testing.T) {