Backport #39075 by @4RH1T3CT0R7
The "New Pull Request" buttons and the `/pulls/new/{branch}` redirect
build their compare link as `{owner}:{branch}`. If a fork and its parent
share an owner, through ALLOW_FORK_INTO_SAME_OWNER, or after a transfer,
that head resolves back to the base repo, so the link compares the base
against itself and 404s on a branch that only exists in the fork.
Switching to `{owner}/{repo}:{branch}` names the head repo
unambiguously, and it's what the compare page's own links already use.
Also clears the 404 in #37649; the archived-parent half of that report
is separate.
Co-authored-by: Artem Lytkin <146867384+4RH1T3CT0R7@users.noreply.github.com>
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