git: extract add_remote() function, and map git2::Error there

I'm going to add check for remote named "git" there.

cli/src/commands/git.rs

@@ -251,12 +251,7 @@ fn cmd_git_remote_add(
     let workspace_command = command.workspace_helper(ui)?;
     let repo = workspace_command.repo();
     let git_repo = get_git_repo(repo.store())?;
-    if git_repo.find_remote(&args.remote).is_ok() {
-        return Err(user_error("Remote already exists"));
-    }
-    git_repo
-        .remote(&args.remote, &args.url)
-        .map_err(|err| user_error(err.to_string()))?;
+    git::add_remote(&git_repo, &args.remote, &args.url)?;
     Ok(())
 }
 

cli/tests/test_git_remotes.rs

@@ -51,6 +51,35 @@ fn test_git_remotes() {
     "###);
 }
 
+#[test]
+fn test_git_remote_add() {
+    let test_env = TestEnvironment::default();
+
+    test_env.jj_cmd_success(test_env.env_root(), &["init", "--git", "repo"]);
+    let repo_path = test_env.env_root().join("repo");
+    test_env.jj_cmd_success(
+        &repo_path,
+        &["git", "remote", "add", "foo", "http://example.com/repo/foo"],
+    );
+    let stderr = test_env.jj_cmd_failure(
+        &repo_path,
+        &[
+            "git",
+            "remote",
+            "add",
+            "foo",
+            "http://example.com/repo/foo2",
+        ],
+    );
+    insta::assert_snapshot!(stderr, @r###"
+    Error: Git remote named 'foo' already exists
+    "###);
+    let stdout = test_env.jj_cmd_success(&repo_path, &["git", "remote", "list"]);
+    insta::assert_snapshot!(stdout, @r###"
+    foo http://example.com/repo/foo
+    "###);
+}
+
 #[test]
 fn test_git_remote_rename() {
     let test_env = TestEnvironment::default();

lib/src/git.rs

@@ -575,6 +575,21 @@ fn is_remote_exists_err(err: &git2::Error) -> bool {
     )
 }
 
+pub fn add_remote(
+    git_repo: &git2::Repository,
+    remote_name: &str,
+    url: &str,
+) -> Result<(), GitRemoteManagementError> {
+    git_repo.remote(remote_name, url).map_err(|err| {
+        if is_remote_exists_err(&err) {
+            GitRemoteManagementError::RemoteAlreadyExists(remote_name.to_owned())
+        } else {
+            GitRemoteManagementError::InternalGitError(err)
+        }
+    })?;
+    Ok(())
+}
+
 pub fn remove_remote(
     mut_repo: &mut MutableRepo,
     git_repo: &git2::Repository,