bootstrap.py: use git rev-list for robustness

Use `git rev-list` instead of `git log` to be more robust against
UI changes in git. Also, use the full email address for bors,
because `--author` uses a substring match.

src/bootstrap/bootstrap.py

@@ -464,16 +464,17 @@ def download_toolchain(self, stage0=True, rustc_channel=None):
             # LLVM more often than necessary.
             #
             # This git command finds that commit SHA, looking for bors-authored
-            # merges that modified src/llvm-project.
+            # commits that modified src/llvm-project or other relevant version
+            # stamp files.
             #
             # This works even in a repository that has not yet initialized
             # submodules.
             top_level = subprocess.check_output([
                 "git", "rev-parse", "--show-toplevel",
             ]).decode(sys.getdefaultencoding()).strip()
             llvm_sha = subprocess.check_output([
-                "git", "log", "--author=bors", "--format=%H", "-n1",
-                "--no-patch", "--first-parent",
+                "git", "rev-list", "--author=bors@rust-lang.org", "-n1",
+                "--first-parent", "HEAD",
                 "--",
                 "{}/src/llvm-project".format(top_level),
                 "{}/src/bootstrap/download-ci-llvm-stamp".format(top_level),
@@ -665,7 +666,7 @@ def maybe_download_ci_toolchain(self):
 
         # Look for a version to compare to based on the current commit.
         # Only commits merged by bors will have CI artifacts.
-        merge_base = ["git", "log", "--author=bors", "--pretty=%H", "-n1"]
+        merge_base = ["git", "rev-list", "--author=bors@rust-lang.org", "-n1", "HEAD"]
         commit = subprocess.check_output(merge_base, universal_newlines=True).strip()
 
         # Warn if there were changes to the compiler or standard library since the ancestor commit.