Make pull merge in on database transaction

[6543]

this is currently not possible because we get a deadlock, at least on sqlite :/ (see comments and commits on #19576)

gitea/services/issue/status.go

Lines 22 to 23 in 92f139d

	// changeStatusCtx changes issue status to open or closed.
	// TODO: if context is not db.DefaultContext we get a deadlock!!!

gitea/services/pull/merge.go

Lines 63 to 65 in 92f139d

	// TODO: make it able to do this in a database session
	mergeCtx := context.Background()
	pr.MergedCommitID, err = rawMerge(mergeCtx, pr, doer, mergeStyle, expectedHeadCommitID, message)

gitea/services/pull/merge.go

Lines 508 to 511 in 92f139d

	// Push back to upstream.
	// TODO: this cause an api call to "/api/internal/hook/post-receive/...",
	// that prevents us from doint the whole merge in one db transaction
	if err := pushCmd.Run(&git.RunOpts{