rustbuild: Check for .git as a dir

Git worktrees have this as a file and typically won't work inside docker
containers, but that's ok, so instead of just checking for existence check for a
directory to see if the git commands will succeed.

src/bootstrap/channel.rs

@@ -69,7 +69,7 @@ pub fn collect(build: &mut Build) {
 
     // If we have a git directory, add in some various SHA information of what
     // commit this compiler was compiled from.
-    if fs::metadata(build.src.join(".git")).is_ok() {
+    if build.src.join(".git").is_dir() {
         let ver_date = output(Command::new("git").current_dir(&build.src)
                                       .arg("log").arg("-1")
                                       .arg("--date=short")

src/bootstrap/dist.rs

@@ -18,7 +18,7 @@
 //! out to `rust-installer` still. This may one day be replaced with bits and
 //! pieces of `rustup.rs`!
 
-use std::fs::{self, File};
+use std::fs::File;
 use std::io::Write;
 use std::path::{PathBuf, Path};
 use std::process::Command;