Keeping consistent between UI and API about combined commit status state and fix some bugs (#34562)

Extract from #34531 

## Move Commit status state to a standalone package

Move the state from `structs` to `commitstatus` package. It also
introduce `CommitStatusStates` so that the combine function could be
used from UI and API logic.

## Combined commit status Changed

This PR will follow Github's combined commit status. Before this PR,
every commit status could be a combined one.
According to
https://docs.github.com/en/rest/commits/statuses?apiVersion=2022-11-28#get-the-combined-status-for-a-specific-reference
> Additionally, a combined state is returned. The state is one of:
> failure if any of the contexts report as error or failure
> pending if there are no statuses or a context is pending
> success if the latest status for all contexts is success

This PR will follow that rule and remove the `NoBetterThan` logic. This
also fixes the inconsistent between UI and API. In the API convert
package, it has implemented this which is different from the UI. It also
fixed the missing `URL` and `CommitURL` in the API.

## `CalcCommitStatus` return nil if there is no commit statuses

The behavior of `CalcCommitStatus` is changed. If the parameter commit
statuses is empty, it will return nil. The reference places should check
the returned value themselves.

modules/commitstatus/commit_status.go

@@ -0,0 +1,81 @@
+// Copyright 2020 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package commitstatus
+
+// CommitStatusState holds the state of a CommitStatus
+// swagger:enum CommitStatusState
+type CommitStatusState string //nolint
+
+const (
+	// CommitStatusPending is for when the CommitStatus is Pending
+	CommitStatusPending CommitStatusState = "pending"
+	// CommitStatusSuccess is for when the CommitStatus is Success
+	CommitStatusSuccess CommitStatusState = "success"
+	// CommitStatusError is for when the CommitStatus is Error
+	CommitStatusError CommitStatusState = "error"
+	// CommitStatusFailure is for when the CommitStatus is Failure
+	CommitStatusFailure CommitStatusState = "failure"
+	// CommitStatusWarning is for when the CommitStatus is Warning
+	CommitStatusWarning CommitStatusState = "warning"
+	// CommitStatusSkipped is for when CommitStatus is Skipped
+	CommitStatusSkipped CommitStatusState = "skipped"
+)
+
+func (css CommitStatusState) String() string {
+	return string(css)
+}
+
+// IsPending represents if commit status state is pending
+func (css CommitStatusState) IsPending() bool {
+	return css == CommitStatusPending
+}
+
+// IsSuccess represents if commit status state is success
+func (css CommitStatusState) IsSuccess() bool {
+	return css == CommitStatusSuccess
+}
+
+// IsError represents if commit status state is error
+func (css CommitStatusState) IsError() bool {
+	return css == CommitStatusError
+}
+
+// IsFailure represents if commit status state is failure
+func (css CommitStatusState) IsFailure() bool {
+	return css == CommitStatusFailure
+}
+
+// IsWarning represents if commit status state is warning
+func (css CommitStatusState) IsWarning() bool {
+	return css == CommitStatusWarning
+}
+
+// IsSkipped represents if commit status state is skipped
+func (css CommitStatusState) IsSkipped() bool {
+	return css == CommitStatusSkipped
+}
+
+type CommitStatusStates []CommitStatusState //nolint
+
+// According to https://docs.github.com/en/rest/commits/statuses?apiVersion=2022-11-28#get-the-combined-status-for-a-specific-reference
+// > Additionally, a combined state is returned. The state is one of:
+// > failure if any of the contexts report as error or failure
+// > pending if there are no statuses or a context is pending
+// > success if the latest status for all contexts is success
+func (css CommitStatusStates) Combine() CommitStatusState {
+	successCnt := 0
+	for _, state := range css {
+		switch {
+		case state.IsError() || state.IsFailure():
+			return CommitStatusFailure
+		case state.IsPending():
+		case state.IsSuccess() || state.IsWarning() || state.IsSkipped():
+			successCnt++
+		}
+	}
+	if successCnt > 0 && successCnt == len(css) {
+		return CommitStatusSuccess
+	}
+	return CommitStatusPending
+}