If TeamCity knows about this commit (i.e. you can see the commit in the TeamCity change log), you can start the right to build from this change. The change log has a small build icon for each commit. Or you can use a custom assembly and select this change from the list.
If TeamCity knows nothing about this commit, then there is currently no way to start the build, see also http://youtrack.jetbrains.com/issue/TW-5610
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