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Backport #26986 by @norohind Fix #20175 Current implementation of API does not allow creating pull requests between branches of the same repo when you specify *namespace* (owner of the repo) in `head` field in http request body. --------- Co-authored-by: norohind <60548839+norohind@users.noreply.github.com> Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>