Use case:

Create a freestyle build project with the following settings:

• This build is parameterized
  Name = BRANCH_SPECIFIER
  Default Value = <leave blank>
  Source Code Management
  Git
• Branch Specifier (blank for default): = $BRANCH_SPECIFIER

Expected:
The build must fail with ERROR because of $BRANCH_SPECIFIER was set as empty and explicit better then implicit. If you want branch spec as "*", then you should set $BRANCH_SPECIFIER was set as "*".

Actually:
$BRANCH_SPECIFIER was set as "**" and job will start in an infinite loop. Build for the build.

Changes inserted in JENKINS-8656 need to be rolled back because they is non logical and misleading.
Commit https://github.com/jenkinsci/git-plugin/commit/dc575dce269020afe79bf0224c906b2520c5438d