title: git merge not creating merge commit, no MERGE_HEAD
body: <p>Using Git 1.8.1.2 on Linux, I have been working on two different branches based on <code>master</code> in parallel (call them A and B), and also created a third branch (call it M) which merges those two branches—so far with no non-merge commits of its own.</p>
  
  <p>After doing some work on one of the “primary” branches (A), I checked out the merge branch (<code>git checkout M</code>), verified that it already had everything in B (<code>git merge B</code> reports <code>Already up-to-date.</code>) , and so I <code>git merge A</code> to pull in the new work on A.</p>
  
  <p>Some merge conflicts are reported, which is expected. I edit the affected files to remove the conflict markers, then <code>git add</code> them. All looking normal so far.</p>
  
  <p>But then when I <code>git commit</code>, the commit message gives no indication of this being a merge. If I proceed, the resulting commit (in the M branch) looks like just a simple commit (atop my prior merge of A and B); <code>git show</code> does not report <code>Merge:</code> parents. Prior to the commit, <code>.git</code> contains <code>MERGE_MODE</code> (empty) but there is no <code>MERGE_HEAD</code>.</p>
  
  <p>I am not using any special merge strategy, so I suppose the default (<code>recursive</code>) is being used. Anyway this does not look like a strategy problem: the contents of the working tree appear exactly like I would expect them to.</p>
  
  <p>If I manually create <code>MERGE_HEAD</code> with the hash of the head of A before committing, the resulting commit looks normal e.g. in <code>gitg</code>, though of course this makes me a little nervous.</p>
  
  <p>What could be going on? Is this a bug in Git—is there ever any reason for <code>MERGE_HEAD</code> not be created after <code>git merge</code> command which did not create a commit? 1.8.4.3 appears to have the same behavior.</p>