Why did my git merge completely blow away my branch's history?
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Aside from my master branch, I have one other branch of interest, let's call it lambda.  I decided to update my lambda branch with changes made from my master branch:

    git checkout lambda
    git merge master

After I did this merge blew away all of my lambda changes.  The `git log` for the lambda branch was an exact duplicate of the master branch.  Fortunately I was able to use `git revert -m 1 <commit id>` to revert the merge and get all of my changes back.

Does anyone know why a merge would wipe out all changes that were committed to my lambda branch?

In case it's useful:  I'm using Git Bash 1.7.10.msysgit.1 on Windows 7.