fix(post): fix target_commitish to be the default branch

target_commitish in GitHub Releases has two different meanings:
* Target commit that a new associated tag will be created on
* Target branch to compute how many "commits to since this release"

target_commitish is usually the default branch (aka master), because the
distance between the release and the latest branch is the primary concern.

Before this change, target_commitish was left to be a hash and the
feature of GitHub Releases that shows how much time passed since the
release is ruined, because no tracking branch is given.

By this change, target_commitish is changed to be the default branch
given in the configuration (options.branch) via edit a release API after
the release and tag are created via crate a release API.

src/post.js

@@ -43,11 +43,23 @@ module.exports = function (config, cb) {
         token: options.githubToken
       })
 
-      github.repos.createRelease(release, function (err) {
+      github.repos.createRelease(release, function (err, res) {
+        if (err) return cb(err)
+        if (options.debug) return cb(null, true, release)
+
+        var editingRelease = {
+          owner: ghRepo[0],
+          repo: ghRepo[1],
+          id: res.id,
+          target_commitish: options.branch
+        }
+
+        github.repos.editRelease(editingRelease, function (err) {
           if (err) return cb(err)
 
           cb(null, true, release)
         })
       })
     })
+  })
 }

test/mocks/github.js

@@ -5,6 +5,9 @@ module.exports = function () {
     },
     repos: {
       createRelease: function (release, cb) {
+        cb(null, {id: 1})
+      },
+      editRelease: function (release, cb) {
         cb(null)
       }
     }