Workaround for Jenkins issue

*steps to reproduce*:
it failed only when set
withMerge policy

``
try {
    stage(“Checkout”) {
       gerrit.gerritPatchsetCheckout([
           credentialsId: CREDENTIALS_ID,
           *withMerge: true*,
       ])
}``

proofs: https://ci.mcp.mirantis.net/view/MAP/job/map-build-component-image/5/console

root cause - incorrect case-resolving in git plugin
related issue: https://issues.jenkins-ci.org/browse/JENKINS-51638
p.s. workaround in comments

Change-Id: I9ffd85a61613eecaeb96fbacb1a28bb6a37cb60f
diff --git a/src/com/mirantis/mk/Gerrit.groovy b/src/com/mirantis/mk/Gerrit.groovy
index 99c91b7..d1c0a1c 100644
--- a/src/com/mirantis/mk/Gerrit.groovy
+++ b/src/com/mirantis/mk/Gerrit.groovy
@@ -76,7 +76,7 @@
 
         // if we need to "merge" code from patchset to GERRIT_BRANCH branch
         if (merge) {
-            scmExtensions.add([$class: 'PreBuildMerge', options: [fastForwardMode: 'FF', mergeRemote: 'gerrit', mergeStrategy: 'default', mergeTarget: gerritBranch]])
+            scmExtensions.add([$class: 'PreBuildMerge', options: [fastForwardMode: 'FF', mergeRemote: 'gerrit', mergeStrategy: 'DEFAULT', mergeTarget: gerritBranch]])
         }
         // we need wipe workspace before checkout
         if (wipe) {
@@ -246,4 +246,4 @@
     def missedParams = requiredParams - config.keySet()
     def badParams = config.subMap(requiredParams).findAll{it.value in [null, '']}.keySet()
     return badParams + missedParams
-}
\ No newline at end of file
+}