cli: annotate: do not panic on reaching initial commit.

As described in jj-vcs#5909, in the case where jj was initialized in a
shallowly cloned repository which was then unshallow'd, jj does not
import the fetched commits that were outside the shallow boundary.

However, it does import the ones after the boundary, which after
unshallowing do not contain the changes made before the boundary.

Therefore, when running annotate in such a case, jj would panic because
it does not find the commit from which a line originates. This simply
sets the empty commit as carrying the blame for that line.

Author: jedesroches

CHANGELOG.md

@@ -45,6 +45,9 @@ to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ### Fixed bugs
 
+* When jj is used with a repository that was shallowly cloned, and then filled
+  up with `git fetch --unshallow`, do not panic on git annotate anymore.
+
 ## [0.27.0] - 2025-03-05
 
 ### Release highlights

cli/src/commands/file/annotate.rs

@@ -16,6 +16,7 @@ use clap_complete::ArgValueCandidates;
 use clap_complete::ArgValueCompleter;
 use jj_lib::annotate::get_annotation_for_file;
 use jj_lib::annotate::FileAnnotation;
+use jj_lib::backend::CommitId;
 use jj_lib::repo::Repo;
 use jj_lib::revset::RevsetExpression;
 use tracing::instrument;
@@ -124,8 +125,9 @@ fn render_file_annotation(
     ui.request_pager();
     let mut formatter = ui.stdout_formatter();
     let mut last_id = None;
+    let default_id = CommitId::from_hex("0000000000000000000000000000000000000000");
     for (line_number, (commit_id, content)) in annotation.lines().enumerate() {
-        let commit_id = commit_id.expect("should reached to the empty ancestor");
+        let commit_id = commit_id.unwrap_or(&default_id);
         let commit = repo.store().get_commit(commit_id)?;
         let first_line_in_hunk = last_id != Some(commit_id);
         let annotation_line = AnnotationLine {