Use traditional try block to close object reader

Support for withCloseable for the AutoCloseable interface, which
ObjectReader implements isn't added until Groovy 2.5. This would have
meant dropping compatibility with Groovy 2.4 and Gradle versions below
5, which isn't necessary.

grgit-core/src/main/groovy/org/ajoberstar/grgit/util/JGitUtil.groovy

@@ -106,8 +106,11 @@ class JGitUtil {
   static Commit convertCommit(Repository repo, RevCommit rev) {
     Map props = [:]
     props.id = ObjectId.toString(rev)
-    repo.jgit.repository.newObjectReader().withCloseable { reader ->
+    def reader = repo.jgit.repository.newObjectReader()
+    try {
       props.abbreviatedId = reader.abbreviate(rev).name()
+    } finally {
+      reader.close()
     }
     PersonIdent committer = rev.committerIdent
     props.committer = new Person(committer.name, committer.emailAddress)