templater: add root keyword

Similar to other boolean flags, such as "working_copy" or "empty".
We could test something like
`"0000000000000000000000000000000000000000".contains(commit_id)`
like I did for myself, but first of all this is ugly, and secondly the root
commit id is not guaranteed to be 40 zeroes as custom backend implementations
could have some other root.

CHANGELOG.md

@@ -31,6 +31,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### New features
 
+* Commit templates now support the `root` keyword, which is `true` for the root
+  commit and `false` for every other commit.
+
 * `jj init --git-repo` now works with bare repositories.
 
 * `jj config edit --user` and `jj config set --user` will now pick a default

cli/src/commit_templater.rs

@@ -306,6 +306,9 @@ fn build_commit_keyword_opt<'repo>(
             let parent_tree = rewrite::merge_commit_trees(repo, &commit.parents()).unwrap();
             commit.tree_id() == parent_tree.id()
         })),
+        "root" => language.wrap_boolean(wrap_fn(property, move |commit| {
+            commit.id() == repo.store().root_commit_id()
+        })),
         _ => return None,
     };
     Some(property)

cli/src/config-schema.json

@@ -175,7 +175,8 @@
                         "is_git_head",
                         "divergent",
                         "hidden",
-                        "conflict"
+                        "conflict",
+                        "root"
                     ]
                 }
             },

docs/templates.md

@@ -34,6 +34,7 @@ The following keywords can be used in `jj log`/`jj obslog` templates.
 * `hidden: Boolean`: True if the commit is not visible (a.k.a. abandoned).
 * `conflict: Boolean`: True if the commit contains merge conflicts.
 * `empty: Boolean`: True if the commit modifies no files.
+* `root: Boolean`: True if the commit is the root commit.
 
 ### Operation keywords