Bail with clear message when file is locked

When attempting to assign a file that is locked by hunks on other 
commits, the code previously printed a debug-style rejection list and 
then misleadingly reported success. 

Instead, fail fast with a user-friendly error that explains the file is
locked to other commit(s) and suggests using git to modify commits or 
move the changes. This prevents confusing output and makes the failure 
reason clear.

Author: schacon

crates/but/src/rub/assign.rs

@@ -3,8 +3,6 @@ use but_workspace::StackId;
 use colored::Colorize;
 use gitbutler_command_context::CommandContext;
 
-use crate::command;
-
 pub(crate) fn assign_file_to_branch(
     ctx: &mut CommandContext,
     path: &str,
@@ -79,7 +77,12 @@ fn do_assignments(
 ) -> anyhow::Result<()> {
     let rejections = but_hunk_assignment::assign(ctx, reqs, None)?;
     if !rejections.is_empty() {
-        command::print(&rejections, false)?;
+        // Don't print the debug output, instead provide a clear error
+        anyhow::bail!(
+            "Cannot assign file - it is locked to {} commit{}. Files are locked when they have changes in commits that conflict with the requested assignment. Use git commands to modify commits or move the changes.",
+            if rejections.len() == 1 { "a" } else { "other" },
+            if rejections.len() == 1 { "" } else { "s" }
+        );
     }
     Ok(())
 }