Add option to 'Rebuild' button to build updated sources

Right now the Rebuild button builds the exact same checkout as another build. This makes it difficult
for the following workflow:
 * run a build on a branch; it fails
 * fix the branch it built
 * hit "rebuild" -> but it builds the original revision
Merge branch 'RebuildOptions' of git://github.com/jaredgrubb/buildbot

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