Support git worktree

[ronanM]

When I go into a git worktree ( https://git-scm.com/docs/git-worktree ), the plugin fails with:

org.eclipse.jgit.api.errors.RefNotFoundException: Ref HEAD can not be resolved
      at org.eclipse.jgit.api.DescribeCommand.setTarget(DescribeCommand.java:137)
      at org.eclipse.jgit.api.DescribeCommand.call(DescribeCommand.java:159)
      at uk.gov.hmrc.gitstamp.GitStamp$.gitStamp(GitStamp.scala:36)
      at uk.gov.hmrc.gitstamp.GitStamp$.gitStamp(GitStamp.scala:30)
      at uk.gov.hmrc.gitstamp.GitStampPlugin$$anonfun$1.apply(GitStampPlugin.scala:29)
      at uk.gov.hmrc.gitstamp.GitStampPlugin$$anonfun$1.apply(GitStampPlugin.scala:28)
      at scala.Function2$$anonfun$tupled$1.apply(Function2.scala:54)
      at scala.Function2$$anonfun$tupled$1.apply(Function2.scala:53)
      at scala.Function1$$anonfun$compose$1.apply(Function1.scala:47)
      at sbt.$tilde$greater$$anonfun$$u2219$1.apply(TypeFunctions.scala:40)
      at sbt.std.Transform$$anon$4.work(System.scala:63)
      at sbt.Execute$$anonfun$submit$1$$anonfun$apply$1.apply(Execute.scala:228)
      at sbt.Execute$$anonfun$submit$1$$anonfun$apply$1.apply(Execute.scala:228)
      at sbt.ErrorHandling$.wideConvert(ErrorHandling.scala:17)