Implement support for rebuilding a revision

[This issue](https://issues.jenkins-ci.org/browse/JENKINS-15306) was filed against the [Rebuild Plugin](https://wiki.jenkins-ci.org/display/JENKINS/Rebuild+Plugin), but more likely can only be accomplished through a change to the Git Plugin.

We find it a common use case to rebuild a revision in our organization, as well.  We prefer to use the Git plugin with [push notification](https://wiki.jenkins-ci.org/display/JENKINS/Git+Plugin#GitPlugin-Pushnotificationfromrepository), which selects the appropriate branch and revision automatically.  This kind of job configuration is not parameterized.  Occasionally, a build is broken due to transient external conditions (e.g. a network outage).  A rebuild is desired to appease code reviewers.

I have tested the proposed patch against Jenkins 1.466 and 1.520.  It co-exists nicely with the Rebuild Plugin.