Fix SHA1s passed to get-changed-trilinos-packages.sh (trilinos#3133, t…

…rilinos#3265)

You need to find the common base commit between the soruce and the target
branch and then use that diff (see trilinos#3265).

Without this change, the auto PR tester will test more package that is it
supposed to.

cmake/std/PullRequestLinuxDriver.sh

@@ -86,6 +86,15 @@ fi
 # advantages to testing newer versions of target instead of older if
 # not all test jobs kick off right away
 
+#-------------------------------
+# Get the merge-base with target
+#-------------------------------
+
+TRILINOS_MERGE_BASE_SHA=`git merge-base ${TRILINOS_SOURCE_SHA} ${TRILINOS_TARGET_SHA}`
+
+# This will give the correct set of changed files for the correct change logic
+# as run by the script get-changed-trilinos-packages.sh
+
 #------------------------------
 # Doing setup for build
 #------------------------------
@@ -161,7 +170,7 @@ echo "CDash Track = ${CDASH_TRACK:?}"
 #-------------------------------------
 rm packageEnables.cmake
 changed_packages_app=Trilinos/commonTools/framework/get-changed-trilinos-packages.sh
-${changed_packages_app} ${TRILINOS_SOURCE_SHA} ${TRILINOS_TARGET_SHA} packageEnables.cmake
+${changed_packages_app} ${TRILINOS_MERGE_BASE_SHA} ${TRILINOS_SOURCE_SHA} packageEnables.cmake
 
 ierror=$?
 if [[ $ierror != 0 ]]; then