Fix github action testing of pull requests on cygwin

The GITHUB_REF head on the remote doesn't reliably contain GITHUB_SHA
when testing pull requests. These PR merge SHAs are likely transient.
Checkout whatever the fetch retrieved from GITHUB_REF instead.

Author: lightsey

.github/workflows/testsuite.yml

@@ -386,7 +386,7 @@ jobs:
         shell: cmd
         run: |
           path %GITHUB_WORKSPACE%\cygwin\bin;%GITHUB_WORKSPACE%\cygwin\usr\bin
-          sh -c "mkdir -p ~; cd ~; echo \"$GITHUB_REPOSITORY\"; git clone -qn \"https://$GITHUB_ACTOR:$GITHUB_TOKEN@github.com/$GITHUB_REPOSITORY\" work ; cd work ; if [ \"$GITHUB_EVENT_NAME\" = pull_request ] ; then git fetch origin \"$GITHUB_REF\" ; fi ; git checkout \"$GITHUB_SHA\""
+          sh -c "mkdir -p ~; cd ~; echo \"$GITHUB_REPOSITORY\"; git clone -qn \"https://$GITHUB_ACTOR:$GITHUB_TOKEN@github.com/$GITHUB_REPOSITORY\" work ; cd work ; if [ \"$GITHUB_EVENT_NAME\" = pull_request ] ; then git fetch origin \"$GITHUB_REF\" && git checkout FETCH_HEAD ; else  git checkout \"$GITHUB_SHA\" ; fi"
       - name: Configure
         shell: cmd
         run: |