tools/ci.sh: For code size build, fetch history of master branch only.

It's not necessary to fetch all branches.
Signed-off-by: Damien George <damien@micropython.org>

tools/ci.sh

@@ -42,7 +42,7 @@ function ci_code_size_build {
     # starts off at either the ref/pull/N/merge FETCH_HEAD, or the current branch HEAD
     git checkout -b pull_request # save the current location
     git remote add upstream https://github.com/micropython/micropython.git
-    git fetch --depth=100 upstream
+    git fetch --depth=100 upstream master
     # build reference, save to size0
     # ignore any errors with this build, in case master is failing
     git checkout `git merge-base --fork-point upstream/master pull_request`