configure nightly branch name in stage0.json

src/bootstrap/config.rs

@@ -226,6 +226,7 @@ pub struct Stage0Config {
     pub artifacts_server: String,
     pub artifacts_with_llvm_assertions_server: String,
     pub git_merge_commit_email: String,
+    pub nightly_branch: String,
 }
 #[derive(Default, Deserialize)]
 #[cfg_attr(test, derive(Clone))]

src/bootstrap/lib.rs

@@ -1280,14 +1280,11 @@ impl Build {
         // Figure out how many merge commits happened since we branched off master.
         // That's our beta number!
         // (Note that we use a `..` range, not the `...` symmetric difference.)
-        let count = output(
-            self.config
-                .git()
-                .arg("rev-list")
-                .arg("--count")
-                .arg("--merges")
-                .arg("refs/remotes/origin/master..HEAD"),
-        );
+        let count =
+            output(self.config.git().arg("rev-list").arg("--count").arg("--merges").arg(format!(
+                "refs/remotes/origin/{}..HEAD",
+                self.config.stage0_metadata.config.nightly_branch
+            )));
         let n = count.trim().parse().unwrap();
         self.prerelease_version.set(Some(n));
         n

src/stage0.json

@@ -3,7 +3,8 @@
     "dist_server": "https://static.rust-lang.org",
     "artifacts_server": "https://ci-artifacts.rust-lang.org/rustc-builds",
     "artifacts_with_llvm_assertions_server": "https://ci-artifacts.rust-lang.org/rustc-builds-alt",
-    "git_merge_commit_email": "bors@rust-lang.org"
+    "git_merge_commit_email": "bors@rust-lang.org",
+    "nightly_branch": "master"
   },
   "__comments": [
     "The configuration above this comment is editable, and can be changed",