templater: add commit.diff().<format>() methods

CHANGELOG.md

@@ -34,6 +34,10 @@ to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 * In git diffs, word-level hunks are now highlighted with underline. See [diff
   colors and styles](docs/config.md#diff-colors-and-styles) for customization.
 
+* New `.diff().<format>()` commit template methods are added. They can be used
+  in order to show diffs conditionally. For example,
+  `if(current_working_copy, diff.summary())`.
+
 * `jj git clone` and `jj git init` with an existing git repository adds the
   default branch of the remote as repository settings for
   `revset-aliases."trunk()"`.`

cli/Cargo.toml

@@ -51,6 +51,7 @@ name = "runner"
 cargo_metadata = { workspace = true }
 
 [dependencies]
+bstr = { workspace = true }
 chrono = { workspace = true }
 clap = { workspace = true }
 clap-markdown = { workspace = true }

cli/src/cli_util.rs

@@ -27,6 +27,7 @@ use std::sync::Arc;
 use std::time::SystemTime;
 use std::{fs, str};
 
+use bstr::ByteVec as _;
 use clap::builder::{
     MapValueParser, NonEmptyStringValueParser, TypedValueParser, ValueParserFactory,
 };
@@ -1063,6 +1064,7 @@ impl WorkspaceCommandHelper {
     pub fn commit_template_language(&self) -> Result<CommitTemplateLanguage<'_>, CommandError> {
         Ok(CommitTemplateLanguage::new(
             self.repo().as_ref(),
+            &self.path_converter,
             self.workspace_id(),
             self.revset_parse_context(),
             self.id_prefix_context()?,
@@ -1084,7 +1086,8 @@ impl WorkspaceCommandHelper {
         let mut output = Vec::new();
         self.write_commit_summary(&mut PlainTextFormatter::new(&mut output), commit)
             .expect("write() to PlainTextFormatter should never fail");
-        String::from_utf8(output).expect("template output should be utf-8 bytes")
+        // Template output is usually UTF-8, but it can contain file content.
+        output.into_string_lossy()
     }
 
     /// Writes one-line summary of the given `commit`.
@@ -1637,7 +1640,8 @@ impl WorkspaceCommandTransaction<'_> {
         let mut output = Vec::new();
         self.write_commit_summary(&mut PlainTextFormatter::new(&mut output), commit)
             .expect("write() to PlainTextFormatter should never fail");
-        String::from_utf8(output).expect("template output should be utf-8 bytes")
+        // Template output is usually UTF-8, but it can contain file content.
+        output.into_string_lossy()
     }
 
     pub fn write_commit_summary(
@@ -1651,6 +1655,7 @@ impl WorkspaceCommandTransaction<'_> {
             .expect("parse error should be confined by WorkspaceCommandHelper::new()");
         let language = CommitTemplateLanguage::new(
             self.tx.repo(),
+            &self.helper.path_converter,
             self.helper.workspace_id(),
             self.helper.revset_parse_context(),
             &id_prefix_context,
@@ -2667,7 +2672,8 @@ pub fn format_template<C: Clone>(ui: &Ui, arg: &C, template: &TemplateRenderer<C
     template
         .format(arg, ui.new_formatter(&mut output).as_mut())
         .expect("write() to vec backed formatter should never fail");
-    String::from_utf8(output).expect("template output should be utf-8 bytes")
+    // Template output is usually UTF-8, but it can contain file content.
+    output.into_string_lossy()
 }
 
 /// CLI command builder and runner.