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.
---
Although GitHub implementation of API allows performing such action and
since Gitea targeting
compatibility with GitHub API I see it as an appropriate change.
I'm proposing a fix to the described problem and test case which covers
this logic.
My use-case just in case:
https://github.com/go-gitea/gitea/issues/20175#issuecomment-1711283022
(cherry picked from commit ed02d1fab85c9b8206c0af84dcfc3792e61609cf)
d74f74a025