Refactor Repo Tag and Branch Target types to a single Ref Target

This commit refactors the existing RepositoryBranchTarget and
RepositoryTagTarget types to a single RepositoryRefTarget since tags and
branches are both refs and the two Target variables represented the
same type anyways. This will also help us move to removing branch
and tag specific options types into a single refs options type.

See ktrysmt#155 and
ktrysmt#153 (comment)

bitbucket.go

@@ -214,6 +214,10 @@ type RepositoryRefOptions struct {
 	BranchFlg bool
 }
 
+type RepositoryRefTarget struct {
+	Hash string `json:"hash"`
+}
+
 type RepositoryBranchOptions struct {
 	Owner    string `json:"owner"`
 	RepoSlug string `json:"repo_slug"`
@@ -226,14 +230,10 @@ type RepositoryBranchOptions struct {
 }
 
 type RepositoryBranchCreationOptions struct {
-	Owner    string                 `json:"owner"`
-	RepoSlug string                 `json:"repo_slug"`
-	Name     string                 `json:"name"`
-	Target   RepositoryBranchTarget `json:"target"`
-}
-
-type RepositoryBranchTarget struct {
-	Hash string `json:"hash"`
+	Owner    string              `json:"owner"`
+	RepoSlug string              `json:"repo_slug"`
+	Name     string              `json:"name"`
+	Target   RepositoryRefTarget `json:"target"`
 }
 
 type RepositoryTagOptions struct {
@@ -251,11 +251,7 @@ type RepositoryTagCreationOptions struct {
 	Owner    string              `json:"owner"`
 	RepoSlug string              `json:"repo_slug"`
 	Name     string              `json:"name"`
-	Target   RepositoryTagTarget `json:"target"`
-}
-
-type RepositoryTagTarget struct {
-	Hash string `json:"hash"`
+	Target   RepositoryRefTarget `json:"target"`
 }
 
 type PullRequestsOptions struct {