Perforce where support edge case broken

It looks like there is a case where we are handed the full path rather than a relative path (in cases where the parent project is a peer to the child project on the filesystem). The fallback to p4 where needs to handle this case.

[DEBUG] Sending changelist:
Change: new

Description:
[maven-release-plugin] prepare for next development iteration

Files:
//depot/external/wicket-1.2.x/wicket-parent/pom.xml
//depot/external/wicket-1.2.x/wicket-parent/C:/src/private/depot/external/wicket-1.2.x/wicket/pom.xml
//depot/external/wicket-1.2.x/wicket-parent/C:/src/private/depot/external/wicket-1.2.x/wicket-extensions/pom.xml