Verify commit signature on HEAD before building from it.

rpki-pbuilder.py

@@ -102,6 +102,7 @@ except (IOError, OSError), e:
 
 run("git", "fetch", "--all", "--prune", cwd = args.git_tree)
 run("git", "pull",                      cwd = args.git_tree)
+run("git", "verify-commit", "HEAD",     cwd = args.git_tree)
 
 source_version = subprocess.check_output((sys.executable, os.path.join(args.git_tree, "buildtools/make-version.py"),
                                           "--build-tag", "--stdout"), cwd = args.git_tree).strip()