Don't print "into HEAD" when merging refs/heads/master

Don't print "into HEAD" when merging refs/heads/master

When MergeMessageFormatter was given a symbolic ref HEAD which points to
refs/heads/master (which is the case when merging a branch in EGit), it
would result in a merge message like the following:

  Merge branch 'a' into HEAD

But it should print the following (as C Git does):

  Merge branch 'a'

The solution is to use the leaf ref when checking for refs/heads/master.

Change-Id: I28ae5713b7e8123a0176fc6d7356e469900e7e97