[beta] Attempt to fix prerelease version calculation

We got rust-lang#47396 merged but it looks like rcs failed to deploy the beta because
when it tried to calculate the beta version its cwd was different. Let's try to
fix this bug and fix auto-deploy by explicitly setting the `current_dir` for git
commands.

src/bootstrap/lib.rs

@@ -787,13 +787,15 @@ impl Build {
                 .arg("ls-remote")
                 .arg("origin")
                 .arg("beta")
+                .current_dir(&self.src)
         );
         let beta = beta.trim().split_whitespace().next().unwrap();
         let master = output(
             Command::new("git")
                 .arg("ls-remote")
                 .arg("origin")
                 .arg("master")
+                .current_dir(&self.src)
         );
         let master = master.trim().split_whitespace().next().unwrap();
 
@@ -802,7 +804,8 @@ impl Build {
             Command::new("git")
                 .arg("merge-base")
                 .arg(beta)
-                .arg(master),
+                .arg(master)
+                .current_dir(&self.src),
         );
         let base = base.trim();
 
@@ -813,7 +816,8 @@ impl Build {
                 .arg("rev-list")
                 .arg("--count")
                 .arg("--merges")
-                .arg(format!("{}...HEAD", base)),
+                .arg(format!("{}...HEAD", base))
+                .current_dir(&self.src),
         );
         let n = count.trim().parse().unwrap();
         self.prerelease_version.set(Some(n));