[JENKINS-57694] Fix calling SpecificRevisionBuildChooser#getDisplayName()

There may be `BuildChooser`s that don't have a `Descriptor` because
they are not meant to be selectable by a user in the UI. Currently
`SpecificRevisionBuildChooser` is such a `BuildChooser`.

This missing `Descriptor` was causing an exception when
`BuildChooser#getDisplayName()` was called within
`GitSCM#compareRemoteRevisionWithImpl()` to log its name.

The `Descriptor` was once already in place but then removed in commit 75e7880
because the `SpecificRevisionBuildChooser` is not meant to be selectable by
a user in the UI.

Therefore the absence of a `Descriptor` has to be handled.

Author: darxriggs

src/main/java/hudson/plugins/git/util/BuildChooser.java

@@ -7,6 +7,7 @@
 import hudson.DescriptorExtensionList;
 import hudson.ExtensionPoint;
 import hudson.model.Describable;
+import hudson.model.Descriptor;
 import jenkins.model.Jenkins;
 import hudson.model.Item;
 import hudson.model.TaskListener;
@@ -46,9 +47,10 @@ public abstract class BuildChooser implements ExtensionPoint, Describable<BuildC
      * @return display name of this build chooser
      */
     public final String getDisplayName() {
-        return getDescriptor().getDisplayName();
+        Descriptor<?> descriptor = Jenkins.get().getDescriptor(getClass());
+        return descriptor != null ? descriptor.getDisplayName() : getClass().getSimpleName();
     }
-    
+
     /**
      * Get a list of revisions that are candidates to be built.
      *