Fix usages of old command API

Author: Kobzol

src/bootstrap/src/core/build_steps/tool.rs

@@ -589,8 +589,7 @@ impl Step for Rustdoc {
                 .arg("--")
                 .arg(librustdoc_src)
                 .arg(rustdoc_src)
-                .run(builder)
-                .is_success();
+                .run(builder);
 
             if !has_changes {
                 let precompiled_rustdoc = builder

src/bootstrap/src/utils/helpers.rs

@@ -534,7 +534,7 @@ pub fn get_closest_merge_base_commit(
     author: &str,
     target_paths: &[PathBuf],
 ) -> Result<String, String> {
-    let mut git = git(source_dir).capture_stdout();
+    let mut git = git(source_dir);
 
     let merge_base = get_git_merge_base(config, source_dir).unwrap_or_else(|_| "HEAD".into());