Adding option to return baseRefOid in pr view

You need to know exact `baseRefOid` so you could show correct diff.
`baseRefName` is not enough sometimes because branch from which PR was
forked might have changes already.

Example usage:
```
gh pr view --json headRefName,headRefOid,number,baseRefName,baseRefOid,reviewDecision
```

api/queries_pr.go

@@ -33,6 +33,7 @@ type PullRequest struct {
 	Closed              bool
 	URL                 string
 	BaseRefName         string
+	BaseRefOid          string
 	HeadRefName         string
 	HeadRefOid          string
 	Body                string

api/query_builder.go

@@ -285,6 +285,7 @@ var PullRequestFields = append(sharedIssuePRFields,
 	"additions",
 	"autoMergeRequest",
 	"baseRefName",
+	"baseRefOid",
 	"changedFiles",
 	"commits",
 	"deletions",

pkg/cmd/pr/view/view_test.go

@@ -32,6 +32,7 @@ func TestJSONFields(t *testing.T) {
 		"author",
 		"autoMergeRequest",
 		"baseRefName",
+		"baseRefOid",
 		"body",
 		"changedFiles",
 		"closed",