cmd: have jj branch list report git-tracking (@git) branches

This doesn't change the way @git branches are stored in `git_refs` as opposed
to inside `BranchTarget` like normal remote-tracking branches. There are
subtle differences in behavior with e.g. `jj branch forget` and I'm not sure
how easy it is to rewrite `jj git import/export` to support a different
way of storage.

I've decided to call these "local-git tracking branches" since they track
branches in the local git repository. "local git-tracking" branches sounds a
bit more natural, but these could be confused with there are no remote
git-tracking branches. If one had the idea these might exist, they would be
confused with remote-tracking branches in the local git repo.

This addresses a portion of #1666

CHANGELOG.md

@@ -101,6 +101,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   now shorter within the default log revset. You can override the default by
   setting the `revsets.short-prefixes` config to a different revset.
 
+* The last seen state of branches in the underlying git repo is now presented by
+  `jj branch list` as a remote called `git` (e.g. `main@git`). Such branches
+  exist in colocated repos or if you use `jj git export`.
+
 ### Fixed bugs
 
 * Modify/delete conflicts now include context lines

lib/src/git.rs

@@ -27,7 +27,7 @@ use crate::op_store::RefTarget;
 use crate::repo::{MutableRepo, Repo};
 use crate::revset;
 use crate::settings::GitSettings;
-use crate::view::RefName;
+use crate::view::{RefName, View};
 
 #[derive(Error, Debug, PartialEq)]
 pub enum GitImportError {
@@ -60,6 +60,16 @@ fn local_branch_name_to_ref_name(branch: &str) -> String {
     format!("refs/heads/{branch}")
 }
 
+// TODO: Eventually, git-tracking branches should no longer be stored in
+// git_refs but with the other remote-tracking branches in BranchTarget. Note
+// that there are important but subtle differences in behavior for, e.g. `jj
+// branch forget`.
+pub fn git_tracking_branches(view: &View) -> impl Iterator<Item = (&str, &RefTarget)> {
+    view.git_refs().iter().filter_map(|(ref_name, target)| {
+        ref_name_to_local_branch_name(ref_name).map(|branch_name| (branch_name, target))
+    })
+}
+
 fn prevent_gc(git_repo: &git2::Repository, id: &CommitId) -> Result<(), git2::Error> {
     // If multiple processes do git::import_refs() in parallel, this can fail to
     // acquire a lock file even with force=true.

src/commands/branch.rs

@@ -3,6 +3,7 @@ use std::collections::BTreeSet;
 use clap::builder::NonEmptyStringValueParser;
 use itertools::Itertools;
 use jujutsu_lib::backend::{CommitId, ObjectId};
+use jujutsu_lib::git::git_tracking_branches;
 use jujutsu_lib::op_store::RefTarget;
 use jujutsu_lib::repo::Repo;
 use jujutsu_lib::revset;
@@ -258,6 +259,28 @@ fn cmd_branch_list(
     let workspace_command = command.workspace_helper(ui)?;
     let repo = workspace_command.repo();
 
+    let mut all_branches = repo.view().branches().clone();
+    for (branch_name, git_tracking_target) in git_tracking_branches(repo.view()) {
+        let branch_target = all_branches.entry(branch_name.to_owned()).or_default();
+        if branch_target.remote_targets.contains_key("git") {
+            // TODO(#1690): There should be a mechanism to prevent importing a
+            // remote named "git" in `jj git import`.
+            // TODO: This is not currently tested
+            writeln!(
+                ui.warning(),
+                "WARNING: Branch {branch_name} has a remote-tracking branch for a remote named \
+                 `git`. Local-git tracking branches for it will not be shown.\nIt is recommended \
+                 to rename that remote, as jj normally reserves the `@git` suffix to denote \
+                 local-git tracking branches."
+            )?;
+        } else {
+            // TODO: `BTreeMap::try_insert` could be used here once that's stabilized.
+            branch_target
+                .remote_targets
+                .insert("git".to_string(), git_tracking_target.clone());
+        }
+    }
+
     let print_branch_target =
         |formatter: &mut dyn Formatter, target: Option<&RefTarget>| -> Result<(), CommandError> {
             match target {
@@ -293,15 +316,12 @@ fn cmd_branch_list(
 
     let mut formatter = ui.stdout_formatter();
     let formatter = formatter.as_mut();
-    for (name, branch_target) in repo.view().branches() {
+
+    for (name, branch_target) in all_branches {
         write!(formatter.labeled("branch"), "{name}")?;
         print_branch_target(formatter, branch_target.local_target.as_ref())?;
 
-        for (remote, remote_target) in branch_target
-            .remote_targets
-            .iter()
-            .sorted_by_key(|(name, _target)| name.to_owned())
-        {
+        for (remote, remote_target) in branch_target.remote_targets.iter() {
             if Some(remote_target) == branch_target.local_target.as_ref() {
                 continue;
             }

tests/test_branch_command.rs

@@ -113,6 +113,8 @@ fn test_branch_forget_glob() {
     "###);
 }
 
+// TODO: Test `jj branch list` with a remote named `git`
+
 fn get_log_output(test_env: &TestEnvironment, cwd: &Path) -> String {
     let template = r#"branches ++ " " ++ commit_id.short()"#;
     test_env.jj_cmd_success(cwd, &["log", "-T", template])

tests/test_git_fetch.rs

@@ -272,6 +272,7 @@ fn test_git_fetch_conflicting_branches_colocated() {
     rem1 (conflicted):
       + f652c32197cf (no description set)
       + 6a21102783e8 message
+      @git (behind by 1 commits): f652c32197cf (no description set)
       @rem1 (behind by 1 commits): 6a21102783e8 message
     "###);
 }

tests/test_git_import_export.rs

@@ -197,6 +197,7 @@ fn test_git_import_move_export_undo() {
     test_env.jj_cmd_success(&repo_path, &["branch", "set", "a"]);
     insta::assert_snapshot!(get_branch_output(&test_env, &repo_path), @r###"
     a: 096dc80da670 (no description set)
+      @git (behind by 1 commits): 230dd059e1b0 (no description set)
     "###);
     insta::assert_snapshot!(test_env.jj_cmd_success(&repo_path, &["git", "export"]), @"");