Backport #24470Fixes#24145
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To solve the bug, I added a "computed" `TargetBehind` field to the
`Release` model, which indicates the target branch of a release. This is
particularly useful if the target branch was deleted in the meantime (or
is empty).
I also did a micro-optimization in `calReleaseNumCommitsBehind`. Instead
of checking that a branch exists and then call `GetBranchCommit`, I
immediately call `GetBranchCommit` and handle the `git.ErrNotExist`
error.
This optimization is covered by the added unit test.
_contributed in the context of @forgejo_
(cherry picked from commit cb7ba8969d)
