Fix pushing SHA1:refs/changes/NNNN

Adding a new patch set to a change using the refs/changes/NNNN syntax
was broken when the label refactoring went in.  The LabelTypes needs
to be initialized in the constructor of ReceiveCommits, not in the
magic branch section.

Change-Id: I4f939d9331015acba733752099c8b28fe06646bc
1 file changed