releases: use specific release-of-tag endpoint

This should be more efficient since we don't need to iterate over the list
of all releases, and hopefully it will also list incomplete assets (though I
can't test this).

Updates github-release#26.

releases.go

@@ -11,6 +11,7 @@ import (
 
 const (
 	RELEASE_LIST_URI    = "/repos/%s/%s/releases"
+	RELEASE_BY_TAG_URI  = "/repos/%s/%s/releases/tags/%s" // /repos/:owner/:repo/releases/tags/:tag
 	RELEASE_LATEST_URI  = "/repos/%s/%s/releases/latest"
 	RELEASE_DATE_FORMAT = "02/01/2006 at 15:04"
 )
@@ -111,19 +112,11 @@ func LatestRelease(user, repo, token string) (*Release, error) {
 	return &releases[latestRelIndex], nil
 }
 
+// ReleaseOfTag returns the Github release corresponding to a git tag, if it
+// exists.
 func ReleaseOfTag(user, repo, tag, token string) (*Release, error) {
-	releases, err := Releases(user, repo, token)
-	if err != nil {
-		return nil, err
-	}
-
-	for _, release := range releases {
-		if release.TagName == tag {
-			return &release, nil
-		}
-	}
-
-	return nil, fmt.Errorf("could not find the release corresponding to tag %s", tag)
+	var release Release
+	return &release, github.Client{Token: token, BaseURL: EnvApiEndpoint}.Get(fmt.Sprintf(RELEASE_BY_TAG_URI, user, repo, tag), &release)
 }
 
 /* find the release-id of the specified tag */