[JENKINS-57694] Fix SpecificRevisionBuildChooser#getDisplayName()

[darxriggs]

JENKINS-57694 - Fix SpecificRevisionBuildChooser#getDisplayName()

old description
All other BuildChoosers already have a Descriptor. 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.

The better way (because a Describable should have a Descriptor) to describe this fact is to overwrite isApplicable() to always return false.

new description
There may be BuildChoosers 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.

Checklist

• I have read the CONTRIBUTING doc
• I have referenced the Jira issue related to my changes in one or more commit messages
• I have added tests that verify my changes
• Unit tests pass locally with my changes
• I have added documentation as necessary
• No Javadoc warnings were introduced with my changes
• No spotbugs warnings were introduced with my changes
• I have interactively tested my changes
• Any dependent changes have been merged and published in upstream modules (like git-client-plugin)

Types of changes

• Bug fix (non-breaking change which fixes an issue)

[jglick]

Amends #709. I guess I am -0. See JIRA. Also IIRC there is another implementation that deliberately omits a Descriptor since it is intended only for programmatic construction.