Previously, using `rebase -r` on the parent of a merge commit
turned it into a non-merge commit. In other words, starting
with
```
o d
|\
o | c
o | b
| o a
|/
o
```
and doing `rebase -r c -d a` resulted in
```
o d
o b
| o c
| o a
|/
o
```
where `d` is no longer a merge commit.
For reference, here's a complete test that passed before this commit (but should NOT pass; see the diff for a test that should pass):
```
#[test]
fn test_rebase_single_revision_merge_parent() {
let test_env = TestEnvironment::default();
test_env.jj_cmd_success(test_env.env_root(), &["init", "repo", "--git"]);
let repo_path = test_env.env_root().join("repo");
create_commit(&test_env, &repo_path, "a", &[]);
create_commit(&test_env, &repo_path, "b", &[]);
create_commit(&test_env, &repo_path, "c", &["b"]);
create_commit(&test_env, &repo_path, "d", &["a", "c"]);
// Test the setup
insta::assert_snapshot!(get_log_output(&test_env, &repo_path), @r###"
@
o d
|\
o | c
o | b
| o a
|/
o
"###);
let stdout = test_env.jj_cmd_success(&repo_path, &["rebase", "-r", "c", "-d", "a"]);
insta::assert_snapshot!(stdout, @r###"
Also rebased 2 descendant commits onto parent of rebased commit
Working copy now at: 3e176b54d680 (no description set)
Added 0 files, modified 0 files, removed 2 files
"###);
insta::assert_snapshot!(get_log_output(&test_env, &repo_path), @r###"
@
o d
| o c
o | b
| o a
|/
o
"###);
}
```
It can be confusing that some commits (typically the working copy)
don't have a description. Let's show a placeholder text in such cases.
I chose the format to match the "(no email configured)" message we
already have.
We didn't have any testing of exit codes on failure, other than
checking that they were not 0. This patch changes that so we always
check. Since we have the special exit code 2 (set by `clap`) for
incorrect command line, I've replaced some testing of error messages
by testing of just the exit code.
As part of this, I also fixed `jj branch --allow-backwards` to
actually require `-r` (it didn't before because having a default value
means the argument is considered always provided).
