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.
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Co-authored-by: norohind <60548839+norohind@users.noreply.github.com>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
(cherry picked from commit 408c92938ba6f38eea1210cae2c485e1f19d4982)
120a173e24