build_mp.py can't fetch MicroPython when the pin is a commit SHA #114

When MicroPython isn't checked out yet, build_mp.py fetches it with

git clone --depth 1 --branch <UPSTREAM> https://github.com/micropython/micropython

(build_mp.py line 178). --branch takes a branch or tag name, not a commit, so when UPSTREAM pins a commit SHA (as it does when the pin is on MicroPython's main between releases) the clone fails and a fresh checkout can't build.

To fix: when UPSTREAM is a 40-character SHA, clone without --branch (or git init + git fetch --depth 1 origin <sha> + git checkout FETCH_HEAD, which GitHub serves for a full SHA), and keep --branch for tags.