use correct git merge message

Author: implausible

src/utils/MergeUtils.js

@@ -1,6 +1,20 @@
 const MergeUtils = {
   getMergeMessage(toBranch, fromBranch) {
-    return `Merged ${fromBranch.shorthand()} into ${toBranch.shorthand()}`;
+    let mergeDecorator;
+    if (fromBranch.isTag()) {
+      mergeDecorator = 'tag';
+    } else if (fromBranch.isRemote()) {
+      mergeDecorator = 'remote-tracking branch';
+    } else {
+      mergeDecorator = 'branch';
+    }
+
+    const message = `Merge ${mergeDecorator} '${fromBranch.shorthand()}'`;
+
+    // https://github.com/git/git/blob/master/builtin/fmt-merge-msg.c#L456-L459
+    return toBranch.shorthand() !== 'master'
+      ? `${message} into ${toBranch.shorthand()}`
+      : message;
   }
 };