[WIP][HOTFIX][SPARK-4123]: Fix bug in PR dependency (all deps. removed issue)

We're seeing a bug sporadically in the new PR dependency comparison test whereby it notes that *all* dependencies are removed. This happens when the current PR is built, but the final, sorted, dependency file is left blank. I believe this is an error either in the way the `git checkout` calls have been or an error within the `mvn` build for that PR (again, likely related to the `git checkout`). As such I've set the checkouts to now force (with `-f` flag) which is more in line with what Jenkins currently does on the initial checkout.

Setting this as a WIP for now to trigger the build process myriad times to see if the issue still arises.

Author: Brennon York <[email protected]>

Closes apache#5443 from brennonyork/HOTFIX2-SPARK-4123 and squashes the following commits:

f2186be [Brennon York] added output for the various git commit refs
3f073d6 [Brennon York] removed the git checkouts piping to dev null
07765a6 [Brennon York] updated the diff logic to reference the filenames rather than hardlink
e3f63c7 [Brennon York] added '-f' to the checkout flags for git
710c8d1 [Brennon York] added 30 minutes to the test benchmark

Author: Brennon York

dev/run-tests-jenkins

@@ -161,6 +161,10 @@ pr_message=""
 # Ensure we save off the current HEAD to revert to
 current_pr_head="`git rev-parse HEAD`"
 
+echo "HEAD:  `git rev-parse HEAD`"
+echo "GHPRB: $ghprbActualCommit"
+echo "SHA1:  $sha1"
+
 # Run pull request tests
 for t in "${PR_TESTS[@]}"; do
   this_test="${FWDIR}/dev/tests/${t}.sh"

dev/tests/pr_new_dependencies.sh

@@ -39,12 +39,12 @@ CURR_CP_FILE="my-classpath.txt"
 MASTER_CP_FILE="master-classpath.txt"
 
 # First switch over to the master branch
-git checkout master &>/dev/null
+git checkout -f master
 # Find and copy all pom.xml files into a *.gate file that we can check
 # against through various `git` changes
 find -name "pom.xml" -exec cp {} {}.gate \;
 # Switch back to the current PR
-git checkout "${current_pr_head}" &>/dev/null
+git checkout -f "${current_pr_head}"
 
 # Check if any *.pom files from the current branch are different from the master
 difference_q=""
@@ -71,7 +71,7 @@ else
     sort > ${CURR_CP_FILE}
 
   # Checkout the master branch to compare against
-  git checkout master &>/dev/null
+  git checkout -f master
 
   ${MVN_BIN} clean package dependency:build-classpath -DskipTests 2>/dev/null | \
     sed -n -e '/Building Spark Project Assembly/,$p' | \
@@ -84,7 +84,7 @@ else
     rev | \
     sort > ${MASTER_CP_FILE}
 
-  DIFF_RESULTS="`diff my-classpath.txt master-classpath.txt`"
+  DIFF_RESULTS="`diff ${CURR_CP_FILE} ${MASTER_CP_FILE}`"
 
   if [ -z "${DIFF_RESULTS}" ]; then
     echo " * This patch does not change any dependencies."