amend: Add more detail to the reflog action

Previously, amend had a customized reflog message, but
it was static and did not have detail on which change was
amended. This made looking through the reflog pretty confusing
if amend was used many times.

Add additional detail to show whether drop/insert was used and
which commit was targeted with the amend. We don't need to
show the new HEAD because this is already in the reflog.

revup/amend.py

@@ -241,8 +241,14 @@ async def get_has_unstaged() -> bool:
         for stack_entry in stack:
             new_commit = await git_ctx.cherry_pick_from_tree(stack_entry, new_commit)
 
+    commit_details = '{}: "{}"'.format(
+        stack[0].commit_id[:8], stack[0].commit_msg.splitlines()[0][:40]
+    )
+    reflog_action_str = "revup amend {}{}".format(
+        "--drop " if args.drop else "--insert " if args.insert else "", commit_details
+    )
     git_env = {
-        "GIT_REFLOG_ACTION": "reset --soft (revup amend)",
+        "GIT_REFLOG_ACTION": reflog_action_str,
     }
     await git_ctx.soft_reset(new_commit, git_env)
     return 0