absorb: move core functionality from cli to lib

cli/src/command_error.rs

@@ -21,6 +21,7 @@ use std::str;
 use std::sync::Arc;
 
 use itertools::Itertools as _;
+use jj_lib::absorb::AbsorbError;
 use jj_lib::backend::BackendError;
 use jj_lib::config::ConfigFileSaveError;
 use jj_lib::config::ConfigGetError;
@@ -590,6 +591,15 @@ impl From<ParseBulkEditMessageError> for CommandError {
     }
 }
 
+impl From<AbsorbError> for CommandError {
+    fn from(err: AbsorbError) -> Self {
+        match err {
+            AbsorbError::Backend(err) => err.into(),
+            AbsorbError::RevsetEvaluation(err) => err.into(),
+        }
+    }
+}
+
 fn find_source_parse_error_hint(err: &dyn error::Error) -> Option<String> {
     let source = err.source()?;
     if let Some(source) = source.downcast_ref() {