Add separate type for PullRequest.Mergeable field

Signed-off-by: Babak K. Shandiz <babak.k.shandiz@gmail.com>

api/queries_pr.go

@@ -16,6 +16,14 @@ type PullRequestAndTotalCount struct {
 	SearchCapped bool
 }
 
+type PullRequestMergeable string
+
+const (
+	PullRequestMergeableConflicting PullRequestMergeable = "CONFLICTING"
+	PullRequestMergeableMergeable   PullRequestMergeable = "MERGEABLE"
+	PullRequestMergeableUnknown     PullRequestMergeable = "UNKNOWN"
+)
+
 type PullRequest struct {
 	ID                  string
 	Number              int
@@ -27,7 +35,7 @@ type PullRequest struct {
 	HeadRefName         string
 	HeadRefOid          string
 	Body                string
-	Mergeable           string
+	Mergeable           PullRequestMergeable
 	Additions           int
 	Deletions           int
 	ChangedFiles        int