feat(config): add jj config get for scripting

The motivating use-case was this `jj signoff` script: https://gist.github.com/thoughtpolice/8f2fd36ae17cd11b8e7bd93a70e31ad6

Which includes lines like this:

```sh
NAME=$(jj config list user.name | awk '{split($0, a, "="); print a[2];}' | tr -d '"')
MAIL=$(jj config list user.email | awk '{split($0, a, "="); print a[2];}' | tr -d '"')
```

There is no reason that we should have to clumsily parse out the config values. This `jj config get` command supports scripting use-cases like this.

CHANGELOG.md

@@ -54,6 +54,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 * `jj git fetch` now supports a `--branch` argument to fetch some of the
   branches only.
 
+* `jj config get` command allows retrieving config values for use in scripting.
+
 * `jj config set` command allows simple config edits like
   `jj config set --repo user.email "somebody@example.com"`
 

src/commands/mod.rs

@@ -189,6 +189,8 @@ impl ConfigArgs {
 enum ConfigSubcommand {
     #[command(visible_alias("l"))]
     List(ConfigListArgs),
+    #[command(visible_alias("g"))]
+    Get(ConfigGetArgs),
     #[command(visible_alias("s"))]
     Set(ConfigSetArgs),
     #[command(visible_alias("e"))]
@@ -208,6 +210,22 @@ struct ConfigListArgs {
     // TODO(#1047): Support ConfigArgs (--user or --repo).
 }
 
+/// Get the value of a given config option.
+///
+/// Unlike `jj config list`, the result of `jj config get` is printed without
+/// extra formatting and therefore is usable in scripting. For example:
+///
+/// $ jj config list user.name
+/// user.name="Martin von Zweigbergk"
+/// $ jj config get user.name
+/// Martin von Zweigbergk
+#[derive(clap::Args, Clone, Debug)]
+#[command(verbatim_doc_comment)]
+struct ConfigGetArgs {
+    #[arg(required = true)]
+    name: String,
+}
+
 /// Update config file to set the given option to a given value.
 #[derive(clap::Args, Clone, Debug)]
 struct ConfigSetArgs {
@@ -1096,6 +1114,7 @@ fn cmd_config(
 ) -> Result<(), CommandError> {
     match subcommand {
         ConfigSubcommand::List(sub_args) => cmd_config_list(ui, command, sub_args),
+        ConfigSubcommand::Get(sub_args) => cmd_config_get(ui, command, sub_args),
         ConfigSubcommand::Set(sub_args) => cmd_config_set(ui, command, sub_args),
         ConfigSubcommand::Edit(sub_args) => cmd_config_edit(ui, command, sub_args),
     }
@@ -1143,6 +1162,43 @@ fn cmd_config_list(
     Ok(())
 }
 
+fn cmd_config_get(
+    ui: &mut Ui,
+    command: &CommandHelper,
+    args: &ConfigGetArgs,
+) -> Result<(), CommandError> {
+    let value = command
+        .settings()
+        .config()
+        .get_string(&args.name)
+        .map_err(|err| match err {
+            config::ConfigError::Type {
+                origin,
+                unexpected,
+                expected,
+                key,
+            } => {
+                let expected = format!("a value convertible to {expected}");
+                // Copied from `impl fmt::Display for ConfigError`. We can't use
+                // the `Display` impl directly because `expected` is required to
+                // be a `'static str`.
+                let mut buf = String::new();
+                use std::fmt::Write;
+                write!(buf, "invalid type: {unexpected}, expected {expected}").unwrap();
+                if let Some(key) = key {
+                    write!(buf, " for key `{key}`").unwrap();
+                }
+                if let Some(origin) = origin {
+                    write!(buf, " in {origin}").unwrap();
+                }
+                CommandError::ConfigError(buf.to_string())
+            }
+            err => err.into(),
+        })?;
+    writeln!(ui, "{value}")?;
+    Ok(())
+}
+
 fn cmd_config_set(
     _ui: &mut Ui,
     command: &CommandHelper,

tests/test_config_command.rs

@@ -438,6 +438,58 @@ fn test_config_edit_repo_outside_repo() {
     "###);
 }
 
+#[test]
+fn test_config_get() {
+    let test_env = TestEnvironment::default();
+    test_env.add_config(
+        r###"
+    [table]
+    string = "some value 1"
+    int = 123
+    list = ["list", "value"]
+    overridden = "foo"
+    "###,
+    );
+    test_env.add_config(
+        r###"
+    [table]
+    overridden = "bar"
+    "###,
+    );
+
+    let stdout = test_env.jj_cmd_failure(test_env.env_root(), &["config", "get", "nonexistent"]);
+    insta::assert_snapshot!(stdout, @r###"
+    Config error: configuration property "nonexistent" not found
+    For help, see https://github.com/martinvonz/jj/blob/main/docs/config.md.
+    "###);
+
+    let stdout = test_env.jj_cmd_success(test_env.env_root(), &["config", "get", "table.string"]);
+    insta::assert_snapshot!(stdout, @r###"
+    some value 1
+    "###);
+
+    let stdout = test_env.jj_cmd_success(test_env.env_root(), &["config", "get", "table.int"]);
+    insta::assert_snapshot!(stdout, @r###"
+    123
+    "###);
+
+    let stdout = test_env.jj_cmd_failure(test_env.env_root(), &["config", "get", "table.list"]);
+    insta::assert_snapshot!(stdout, @r###"
+    Config error: invalid type: sequence, expected a value convertible to a string
+    For help, see https://github.com/martinvonz/jj/blob/main/docs/config.md.
+    "###);
+
+    let stdout = test_env.jj_cmd_failure(test_env.env_root(), &["config", "get", "table"]);
+    insta::assert_snapshot!(stdout, @r###"
+    Config error: invalid type: map, expected a value convertible to a string
+    For help, see https://github.com/martinvonz/jj/blob/main/docs/config.md.
+    "###);
+
+    let stdout =
+        test_env.jj_cmd_success(test_env.env_root(), &["config", "get", "table.overridden"]);
+    insta::assert_snapshot!(stdout, @"bar");
+}
+
 fn find_stdout_lines(keyname_pattern: &str, stdout: &str) -> String {
     let key_line_re = Regex::new(&format!(r"(?m)^{keyname_pattern}=.*$")).unwrap();
     key_line_re