create_release.py: Push resulting branch to origin

Also, set the upstream when creating the new branch.

docs/process.md

@@ -107,8 +107,8 @@ How:
    name release. Running this
    [`scripts/create_release.py`][create_release_emsdk] script will update
    [emscripten-releases-tags.json][emscripten_releases_tags], adding a new
-   version. The script will create a new git branch that can be uploaded as a
-   PR. An example of this PR is emscripten-core/emsdk#1071.
+   version. The script will create a new local git branch and push it up to
+   ``origin``.  An example of this PR is emscripten-core/emsdk#1071.
 1. [Tag][emsdk_tags] the `emsdk` repo with the new version number, on the commit
    that does the update, after it lands on main.
 1. [Tag][emscripten_tags] the `emscripten` repo with the new version number, on

tools/maint/create_release.py

@@ -59,15 +59,15 @@ def main():
   branch_name = 'version_' + release_version
 
   # Create a new git branch
-  subprocess.check_call(['git', 'checkout', '-b', branch_name], cwd=root_dir)
+  subprocess.check_call(['git', 'checkout', '-b', branch_name, 'origin/main'], cwd=root_dir)
 
   # Create auto-generated changes to the new git branch
   subprocess.check_call(['git', 'add', '-u', '.'], cwd=root_dir)
   subprocess.check_call(['git', 'commit', '-m', f'Mark {release_version} as released'], cwd=root_dir)
+  print('New release created in branch: `%s`' % branch_name)
 
-  print('New relase created in branch: `%s`' % branch_name)
-
-  # TODO(sbc): Maybe create the tag too, and even push both to `origin`?
+  # Push new branch to origin
+  subprocess.check_call(['git', 'push', 'origin', branch_name], cwd=root_dir)
   return 0