repo: add a specific error type for MutableRepo::edit()

The new type is just an enum version of `RewriteRootCommit`.  I'll add
another variant soon.

lib/src/repo.rs

@@ -764,9 +764,10 @@ impl MutableRepo {
         &mut self,
         workspace_id: WorkspaceId,
         commit: &Commit,
-    ) -> Result<(), RewriteRootCommit> {
+    ) -> Result<(), EditCommitError> {
         self.leave_commit(&workspace_id);
         self.set_wc_commit(workspace_id, commit.id().clone())
+            .map_err(|RewriteRootCommit| EditCommitError::RewriteRootCommit)
     }
 
     fn leave_commit(&mut self, workspace_id: &WorkspaceId) {
@@ -1110,6 +1111,13 @@ impl MutableRepo {
 #[error("Cannot rewrite the root commit")]
 pub struct RewriteRootCommit;
 
+/// Error from attempts to edit a commit
+#[derive(Debug, Error)]
+pub enum EditCommitError {
+    #[error("Cannot rewrite the root commit")]
+    RewriteRootCommit,
+}
+
 #[derive(Debug, Error)]
 #[error("Cannot access {path}")]
 pub struct PathError {

src/cli_util.rs

@@ -37,7 +37,8 @@ use jujutsu_lib::op_heads_store::{self, OpHeadResolutionError, OpHeadsStore};
 use jujutsu_lib::op_store::{OpStore, OpStoreError, OperationId, WorkspaceId};
 use jujutsu_lib::operation::Operation;
 use jujutsu_lib::repo::{
-    MutableRepo, ReadonlyRepo, RepoLoader, RepoRef, RewriteRootCommit, StoreFactories,
+    EditCommitError, MutableRepo, ReadonlyRepo, RepoLoader, RepoRef, RewriteRootCommit,
+    StoreFactories,
 };
 use jujutsu_lib::repo_path::{FsPathParseError, RepoPath};
 use jujutsu_lib::revset::{
@@ -117,6 +118,12 @@ impl From<RewriteRootCommit> for CommandError {
     }
 }
 
+impl From<EditCommitError> for CommandError {
+    fn from(err: EditCommitError) -> Self {
+        CommandError::InternalError(format!("Failed to edit a commit: {err}"))
+    }
+}
+
 impl From<BackendError> for CommandError {
     fn from(err: BackendError) -> Self {
         user_error(format!("Unexpected error from backend: {err}"))