workspace: make recovery commit empty instead of deleting everything

The recovery commit we create when we run into a stale working copy
with a missing operation currently has an empty tree. Our commit
backend at Google creates an index of which files changed in each
commit. That gets really expensive when a commit deletes all files in
the repo, as these recovery commits do. So for our backend, it is much
better to make the recovery commit empty instead. That's what this
patch does.

It almost doesn't matter functionally what tree we use for it since we
don't care much about the current tree when snapshotting the working
copy. It does matter in a few cases, however. One case is for
conflicts. In that case, it's likely better to use the recovery
commit's parent as base tree (as we do by making the recovery commit
empty) than to use an empty tree, as that would guarantee that all
conflicts would be considered resolved. (Side note: perhaps we should
start looking at the current commit's parent instead of looking at the
current commit when snapshotting, but that's a topic for another day.)

cli/src/commands/workspace.rs

@@ -307,14 +307,17 @@ fn create_and_check_out_recovery_commit(
     let workspace_id = workspace_command.workspace_id().clone();
     let mut tx = workspace_command.start_transaction().into_inner();
 
-    let tree_id = workspace_command.repo().store().empty_merged_tree_id();
     let (mut locked_workspace, commit) =
         workspace_command.unchecked_start_working_copy_mutation()?;
     let commit_id = commit.id();
 
     let mut_repo = tx.mut_repo();
     let new_commit = mut_repo
-        .new_commit(command.settings(), vec![commit_id.clone()], tree_id.clone())
+        .new_commit(
+            command.settings(),
+            vec![commit_id.clone()],
+            commit.tree_id().clone(),
+        )
         .write()?;
     mut_repo.set_wc_commit(workspace_id, new_commit.id().clone())?;
     let repo = tx.commit("recovery commit");

cli/tests/test_workspaces.rs

@@ -467,12 +467,12 @@ fn test_workspaces_current_op_discarded_by_other() {
     let (stdout, stderr) = test_env.jj_cmd_ok(&secondary_path, &["workspace", "update-stale"]);
     insta::assert_snapshot!(stderr, @r###"
     Failed to read working copy's current operation; attempting recovery. Error message from read attempt: Object 47f1ad5e1adfaa1e9863181e6e45ae12b9db553e82bde82e278c0c6288053c833b4039a5baf2415871964bcdcd4b7875692979e66f2ec18bfc3896355091cf53 of type operation not found
-    Created and checked out recovery commit 9d040f9a433c
+    Created and checked out recovery commit df3e46148439
     "###);
     insta::assert_snapshot!(stdout, @"");
 
     insta::assert_snapshot!(get_log_output(&test_env, &main_path), @r###"
-    ◉  e5b7d155229f587a96678aaa23b771c694f5bb8ad690e1312b90dd3c73b01ab26ea0a2946aa2760f3c672f1587f2742718a6596c46c3a3b881122906164c081a secondary@
+    ◉  54400d0c58b790c53de57a1c28090d23739010b300ac17cb47c26d1e48d41bb6583c94a5e4e71b32bbe1e50190c67141525e92aba9ae7b7a413ed1c989629874 secondary@
     ◉  4278b78fb503846224c4328785f8557558aab32b1e3d6ad6b343e94575dd5c67492acea9aa1c417817b4943e78547877e031208a51f8eadcd1ac55d6ebe77983
     │ @  6dc8f254cd3cbe5c086fb99adc66724303d7ceb12790e585e5946940dffca60dbb928b852e3c309b20dee351df5897f49b83c89d583a756fd736772b3e5f206a default@
     ├─╯
@@ -484,16 +484,16 @@ fn test_workspaces_current_op_discarded_by_other() {
     insta::assert_snapshot!(stdout, @r###"
     Working copy changes:
     A file
-    Working copy : znkkpsqq e5b7d155 (no description set)
+    Working copy : znkkpsqq 54400d0c (no description set)
     Parent commit: pmmvwywv 4278b78f (empty) (no description set)
     "###);
 
     let (stdout, stderr) = test_env.jj_cmd_ok(&secondary_path, &["obslog"]);
     insta::assert_snapshot!(stderr, @"");
     insta::assert_snapshot!(stdout, @r###"
-    @  znkkpsqq [email protected] 2001-02-03 04:05:16.000 +07:00 secondary@ e5b7d155
+    @  znkkpsqq [email protected] 2001-02-03 04:05:16.000 +07:00 secondary@ 54400d0c
     │  (no description set)
-    ◉  znkkpsqq hidden [email protected] 2001-02-03 04:05:16.000 +07:00 9d040f9a
+    ◉  znkkpsqq hidden [email protected] 2001-02-03 04:05:16.000 +07:00 df3e4614
        (empty) (no description set)
     "###);
 }