make getCheckoutInfo more reusable

vars/getCheckoutInfo.groovy

@@ -1,28 +1,26 @@
 def call() {
 
   def repoInfo = [
-    branch: env.ghprbSourceBranch ?: params.branch_specifier ?: 'missing branch_specifier',
+    branch: env.ghprbSourceBranch ?: params.branch_specifier ?: null,
     targetBranch: env.ghprbTargetBranch,
   ]
 
-  dir("kibana") {
-    repoInfo.commit = sh(
-      script: "git rev-parse HEAD",
-      label: "determining checked out sha",
+  repoInfo.commit = sh(
+    script: "git rev-parse HEAD",
+    label: "determining checked out sha",
+    returnStdout: true
+  ).trim()
+
+  if (repoInfo.targetBranch) {
+    sh(
+      script: "git fetch origin ${repoInfo.targetBranch}",
+      label: "fetch latest from '${repoInfo.targetBranch}' at origin"
+    )
+    repoInfo.mergeBase = sh(
+      script: "git merge-base HEAD FETCH_HEAD",
+      label: "determining merge point with '${repoInfo.targetBranch}' at origin",
       returnStdout: true
     ).trim()
-
-    if (repoInfo.targetBranch) {
-      sh(
-        script: "git fetch https://github.com/elastic/kibana.git ${repoInfo.targetBranch}",
-        label: "fetch latest from '${repoInfo.targetBranch}' at https://github.com/elastic/kibana.git"
-      )
-      repoInfo.mergeBase = sh(
-        script: "git merge-base HEAD FETCH_HEAD",
-        label: "determining merge point with '${repoInfo.targetBranch}' at https://github.com/elastic/kibana.git",
-        returnStdout: true
-      ).trim()
-    }
   }
 
   print "repoInfo: ${repoInfo}"

vars/workers.groovy

@@ -59,7 +59,11 @@ def base(Map params, Closure closure) {
         checkout scm
       }
 
-      checkoutInfo = getCheckoutInfo()
+      def checkoutInfo
+      dir("kibana") {
+        checkoutInfo = getCheckoutInfo()
+      }
+
       ciStats.reportGitInfo(
         checkoutInfo.branch,
         checkoutInfo.commit,