mock GitUtils

if we don't mock the GitUtils here changes inside GitUtils affecting the git commands
issued at the command line requires changes here. In fact we have to react on
implementation details of the GitUtils here. It is better to be independent from that
implementation details here since this leads to more focused and smaller commits.

test/groovy/ArtifactSetVersionTest.groovy

@@ -1,9 +1,13 @@
 #!groovy
+
 import org.junit.Before
 import org.junit.Rule
 import org.junit.Test
 import org.junit.rules.ExpectedException
 import org.junit.rules.RuleChain
+
+import com.sap.piper.GitUtils
+
 import util.BasePiperTest
 import util.JenkinsDockerExecuteRule
 import util.JenkinsEnvironmentRule
@@ -26,6 +30,16 @@ import static org.junit.Assert.assertEquals
 class ArtifactSetVersionTest extends BasePiperTest {
     Map dockerParameters
 
+    def GitUtils gitUtils = new GitUtils() {
+        boolean insideWorkTree() {
+            return true
+        }
+
+        String getGitCommitIdOrNull() {
+            return 'testCommitId'
+        }
+    }
+
     def sshAgentList = []
 
     private ExpectedException thrown = ExpectedException.none()
@@ -61,10 +75,8 @@ class ArtifactSetVersionTest extends BasePiperTest {
             return closure()
         })
 
-        jscr.setReturnValue('git rev-parse HEAD', 'testCommitId')
         jscr.setReturnValue("date --universal +'%Y%m%d%H%M%S'", '20180101010203')
         jscr.setReturnValue('git diff --quiet HEAD', 0)
-        jscr.setReturnValue('git rev-parse --is-inside-work-tree 1>/dev/null 2>&1', 0)
 
         helper.registerAllowedMethod('fileExists', [String.class], {true})
     }