Don't recreate workspace when using Repository Sharing

When using Repository Sharing, the working copy is always recloned (from
the slave cache), which can take a long time on large repos.

This fixes the logic used to detect whether a workspace copy can be
reused to support Repository Sharing.

JENKINS-15829
Merge pull request #50 from blt04/fix-shared-working-copy-update

[FIXED JENKINS-15829] Don't recreate workspace when using Repository Sharing