cli: change default log revset to not include all tagged heads

The default immutable_heads() includes tags(), which makes sense, but computing
heads(tags()) can be expensive because the tags() set is usually sparse. For
example, "jj bench revset 'heads(tags())'" took 157ms in my linux stable
mirror. We can of course optimize the heads evaluation by using bit set or
segmented index, but the query includes many historical heads if the repository
has per-release branches, which are uninteresting anyway. So, this patch
replaces heads(immutable_heads()) with trunk().

The reason we include heads(immutable_heads()) is to mitigate the following
problem. Suppose trunk() is the branch to be based off, I think using trunk()
here is pretty good.

```
A   B
*---*----* trunk() ⊆ immutable_heads()
     \
      * C
```
#2247 (comment)

CHANGELOG.md

@@ -22,6 +22,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 * Dropped support for the "legacy" graph-drawing style. Use "ascii" for a very
   similar result.
 
+* The default log output no longer lists all tagged heads. Set `revsets.log =
+  "@ | ancestors(immutable_heads().., 2) | heads(immutable_heads())"` to restore
+  the old behavior.
+
 * Dropped support for the deprecated `:` revset operator. Use `::` instead.
 
 ### New features

cli/src/commands/log.rs

@@ -30,8 +30,7 @@ use crate::ui::Ui;
 #[derive(clap::Args, Clone, Debug)]
 pub(crate) struct LogArgs {
     /// Which revisions to show. Defaults to the `revsets.log` setting, or
-    /// `@ | ancestors(immutable_heads().., 2) | heads(immutable_heads())` if
-    /// it is not set.
+    /// `@ | ancestors(immutable_heads().., 2) | trunk()` if it is not set.
     #[arg(long, short)]
     revisions: Vec<RevisionArg>,
     /// Show commits modifying the given paths

cli/src/config-schema.json

@@ -303,7 +303,7 @@
                 "log": {
                     "type": "string",
                     "description": "Default set of revisions to show when no explicit revset is given for jj log and similar commands",
-                    "default": "@ | ancestors(immutable_heads().., 2) | heads(immutable_heads())"
+                    "default": "@ | ancestors(immutable_heads().., 2) | trunk()"
                 },
                 "short-prefixes": {
                     "type": "string",

cli/tests/cli-reference@.md.snap

@@ -1002,7 +1002,7 @@ Show commit history
 
 ###### **Options:**
 
-* `-r`, `--revisions <REVISIONS>` — Which revisions to show. Defaults to the `revsets.log` setting, or `@ | ancestors(immutable_heads().., 2) | heads(immutable_heads())` if it is not set
+* `-r`, `--revisions <REVISIONS>` — Which revisions to show. Defaults to the `revsets.log` setting, or `@ | ancestors(immutable_heads().., 2) | trunk()` if it is not set
 * `--reversed` — Show revisions in the opposite order (older revisions first)
 
   Possible values: `true`, `false`

cli/tests/test_immutable_commits.rs

@@ -64,7 +64,7 @@ fn test_rewrite_immutable_generic() {
     insta::assert_snapshot!(stderr, @r###"
     Config error: Invalid `revsets.short-prefixes`:  --> 1:31
       |
-    1 | @ | ancestors(immutable_heads().., 2) | heads(immutable_heads())
+    1 | @ | ancestors(immutable_heads().., 2) | trunk()
       |                               ^
       |
       = Invalid arguments to revset function "immutable_heads": Expected 1 arguments
@@ -93,8 +93,9 @@ fn test_rewrite_immutable_commands() {
     test_env.jj_cmd_ok(&repo_path, &["branch", "create", "main"]);
     test_env.jj_cmd_ok(&repo_path, &["new", "description(b)"]);
     test_env.add_config(r#"revset-aliases."immutable_heads()" = "main""#);
+    test_env.add_config(r#"revset-aliases."trunk()" = "main""#);
 
-    // Log shows mutable commits and immutable heads by default
+    // Log shows mutable commits, their parents, and trunk() by default
     let stdout = test_env.jj_cmd_success(&repo_path, &["log"]);
     insta::assert_snapshot!(stdout, @r###"
     @  yqosqzyt [email protected] 2001-02-03 04:05:13.000 +07:00 3f89addf

lib/src/settings.rs

@@ -209,9 +209,7 @@ impl UserSettings {
             // For compatibility with old config files (<0.8.0)
             self.config
                 .get_string("ui.default-revset")
-                .unwrap_or_else(|_| {
-                    "@ | ancestors(immutable_heads().., 2) | heads(immutable_heads())".to_string()
-                })
+                .unwrap_or_else(|_| "@ | ancestors(immutable_heads().., 2) | trunk()".to_string())
         })
     }