ci/ci_commit_formatting_run: Ensure checks work on older branches.

tools/ci.sh

@@ -33,8 +33,10 @@ function ci_code_formatting_run {
 
 function ci_commit_formatting_run {
     git remote add upstream https://github.com/micropython/micropython.git
-    git fetch --depth=100 upstream  master
-    # For a PR, upstream/master..HEAD ends with a merge commit into master, exlude that one.
+    git fetch --depth=100 upstream master
+    # If the common ancestor commit hasn't been found, fetch more
+    git merge-base upstream/master HEAD || git fetch --unshallow upstream master
+    # For a PR, upstream/master..HEAD ends with a merge commit into master, exclude that one.
     tools/verifygitlog.py -v upstream/master..HEAD --no-merges
 }
 
@@ -53,6 +55,8 @@ function ci_code_size_build {
     git checkout -b pull_request # save the current location
     git remote add upstream https://github.com/micropython/micropython.git
     git fetch --depth=100 upstream master
+    # If the common ancestor commit hasn't been found, fetch more
+    git merge-base upstream/master HEAD || git fetch --unshallow 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`