Prevent extended tests run unexpectedly in appveyor

Reason turns out that "git log -2" is picking up a merge
commit and a random commit message from the master branch.

Restore the expected behavior by using
git log -1 $env:APPVEYOR_PULL_REQUEST_HEAD_COMMIT

Reviewed-by: Tomas Mraz <tmraz@fedoraproject.org>
(Merged from https://github.com/openssl/openssl/pull/11981)

appveyor.yml

@@ -46,7 +46,8 @@ before_build:
     - cd ..
     - ps: >-
         if (-not $env:APPVEYOR_PULL_REQUEST_NUMBER`
-            -or (&git log -2 | Select-String "\[extended tests\]") ) {
+            -or (&git log -1 $env:APPVEYOR_PULL_REQUEST_HEAD_COMMIT |
+                 Select-String "\[extended tests\]") ) {
             $env:EXTENDED_TESTS="yes"
         }