cli: show Git HEAD in log output

It's useful to know which commit is checked out in the underlying Git
repo (if there is one), so let's show that. This patch indicates that
commit with `HEAD@git` in the log output. It's probably not very
useful when the Git repo is "internal" (i.e. stored inside `.jj/`),
because then it's unlikely to change often. I therefore considered not
showing it when the Git repo is internal. However, it turned out that
`HEAD` points to a non-existent branch in the repo I use, so it won't
get imported anyway (by the function added in the previous patch). We
can always review this decision later.

This is part of #44.

lib/src/repo.rs

@@ -67,6 +67,13 @@ pub enum RepoRef<'a> {
 }
 
 impl<'a> RepoRef<'a> {
+    pub fn base_repo(&self) -> &ReadonlyRepo {
+        match self {
+            RepoRef::Readonly(repo) => repo,
+            RepoRef::Mutable(repo) => repo.base_repo.as_ref(),
+        }
+    }
+
     pub fn store(&self) -> &Arc<Store> {
         match self {
             RepoRef::Readonly(repo) => repo.store(),

src/commands.rs

@@ -2195,6 +2195,7 @@ fn log_template(settings: &UserSettings) -> String {
             " " label("timestamp", author.timestamp())
             " " branches
             " " tags
+            if(is_git_head, label("git_head", " HEAD@git"))
             if(divergent, label("divergent", " divergent"))
             if(conflict, label("conflict", " conflict"))
             "\n"

src/formatter.rs

@@ -92,6 +92,7 @@ fn config_colors(user_settings: &UserSettings) -> HashMap<String, String> {
     result.insert(String::from("branches"), String::from("magenta"));
     result.insert(String::from("tags"), String::from("magenta"));
     result.insert(String::from("git_refs"), String::from("magenta"));
+    result.insert(String::from("git_head"), String::from("magenta"));
     result.insert(String::from("divergent"), String::from("red"));
     result.insert(String::from("conflict"), String::from("red"));
 

src/template_parser.rs

@@ -25,9 +25,9 @@ use crate::formatter::PlainTextFormatter;
 use crate::templater::{
     AuthorProperty, BranchProperty, ChangeIdProperty, CommitIdKeyword, CommitterProperty,
     ConditionalTemplate, ConflictProperty, ConstantTemplateProperty, CurrentCheckoutProperty,
-    DescriptionProperty, DivergentProperty, DynamicLabelTemplate, GitRefsProperty, LabelTemplate,
-    ListTemplate, LiteralTemplate, OpenProperty, StringPropertyTemplate, TagProperty, Template,
-    TemplateFunction, TemplateProperty,
+    DescriptionProperty, DivergentProperty, DynamicLabelTemplate, GitRefsProperty,
+    IsGitHeadProperty, LabelTemplate, ListTemplate, LiteralTemplate, OpenProperty,
+    StringPropertyTemplate, TagProperty, Template, TemplateFunction, TemplateProperty,
 };
 
 #[derive(Parser)]
@@ -242,6 +242,7 @@ fn parse_commit_keyword<'a>(repo: RepoRef<'a>, pair: Pair<Rule>) -> (Property<'a
         "branches" => Property::String(Box::new(BranchProperty { repo })),
         "tags" => Property::String(Box::new(TagProperty { repo })),
         "git_refs" => Property::String(Box::new(GitRefsProperty { repo })),
+        "is_git_head" => Property::Boolean(Box::new(IsGitHeadProperty::new(repo))),
         "divergent" => Property::Boolean(Box::new(DivergentProperty::new(repo))),
         "conflict" => Property::Boolean(Box::new(ConflictProperty)),
         name => panic!("unexpected identifier: {}", name),

src/templater.rs

@@ -288,6 +288,22 @@ impl TemplateProperty<Commit, String> for GitRefsProperty<'_> {
     }
 }
 
+pub struct IsGitHeadProperty<'a> {
+    repo: RepoRef<'a>,
+}
+
+impl<'a> IsGitHeadProperty<'a> {
+    pub fn new(repo: RepoRef<'a>) -> Self {
+        Self { repo }
+    }
+}
+
+impl TemplateProperty<Commit, bool> for IsGitHeadProperty<'_> {
+    fn extract(&self, context: &Commit) -> bool {
+        self.repo.view().git_head().as_ref() == Some(context.id())
+    }
+}
+
 pub struct DivergentProperty {
     divergent_changes: HashSet<ChangeId>,
 }