Adds support for composing without a base commit #4656
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Closes #4637
Strategy here is to create an empty base commit without touching the working tree by using temporary index and commit-tree, then pointing HEAD to it, and then there is a commit that we can stash on temporarily. We then proceed to create the composer commits as patches and commit them with a similar strategy, except they are on another temporary index off the root sha. Then when we reset, our stash and empty commit are orphaned but still accessible. We pop the stash as normal and then let garbage collection eliminate the temporary empty commit we created.