git_backend: classify "merge with root" as user error

Perhaps, there will be more error types that hold BackendError internally, but
this change is good enough to handle a merge error.

cli/src/command_error.rs

@@ -238,7 +238,10 @@ impl From<CheckOutCommitError> for CommandError {
 
 impl From<BackendError> for CommandError {
     fn from(err: BackendError) -> Self {
-        internal_error_with_message("Unexpected error from backend", err)
+        match &err {
+            BackendError::Unsupported(_) => user_error(err),
+            _ => internal_error_with_message("Unexpected error from backend", err),
+        }
     }
 }
 
@@ -305,7 +308,11 @@ want this file to be snapshotted. Otherwise add it to your `.gitignore` file."#,
 
 impl From<TreeMergeError> for CommandError {
     fn from(err: TreeMergeError) -> Self {
-        internal_error_with_message("Merge failed", err)
+        let kind = match &err {
+            TreeMergeError::BackendError(BackendError::Unsupported(_)) => CommandErrorKind::User,
+            _ => CommandErrorKind::Internal,
+        };
+        CommandError::with_message(kind, "Merge failed", err)
     }
 }
 

cli/tests/test_abandon_command.rs

@@ -314,9 +314,9 @@ fn test_bug_2600_rootcommit_special_case() {
     "###);
 
     // Now, the test
-    let stderr = test_env.jj_cmd_internal_error(&repo_path, &["abandon", "base"]);
+    let stderr = test_env.jj_cmd_failure(&repo_path, &["abandon", "base"]);
     insta::assert_snapshot!(stderr, @r###"
-    Internal error: Merge failed
+    Error: Merge failed
     Caused by: The Git backend does not support creating merge commits with the root commit as one of the parents.
     "###);
 }

lib/src/backend.rs

@@ -200,6 +200,10 @@ pub enum BackendError {
     },
     #[error(transparent)]
     Other(Box<dyn std::error::Error + Send + Sync>),
+    /// A valid operation attempted, but failed because it isn't supported by
+    /// the particular backend.
+    #[error("{0}")]
+    Unsupported(String),
 }
 
 pub type BackendResult<T> = Result<T, BackendError>;

lib/src/git_backend.rs

@@ -1121,10 +1121,10 @@ impl Backend for GitBackend {
                 // there are no other parents since Git cannot represent a merge between a root
                 // commit and another commit.
                 if contents.parents.len() > 1 {
-                    return Err(BackendError::Other(
+                    return Err(BackendError::Unsupported(
                         "The Git backend does not support creating merge commits with the root \
                          commit as one of the parents."
-                            .into(),
+                            .to_owned(),
                     ));
                 }
             } else {
@@ -1702,7 +1702,7 @@ mod tests {
         commit.parents = vec![first_id, backend.root_commit_id().clone()];
         assert_matches!(
             backend.write_commit(commit, None),
-            Err(BackendError::Other(err)) if err.to_string().contains("root commit")
+            Err(BackendError::Unsupported(message)) if message.contains("root commit")
         );
     }