This fixes two problems with the "amend commit with staged changes" command:
1. Amending to a fixup commit didn't work (this would create a commmit with the
title "fixup! fixup! original title" and keep that at the top of the branch)
2. Unrelated fixup commits would be squashed too.
The added integration test verifies that both of these problems are fixed.
Previously we would have tried to do the rebase, resulting in a long and
somewhat cryptic error message from git; now we check ourselves and show a less
intimidating message.
The patch contains changes to two files; the first one conflicts, the second
doesn't. Note how it only applies changes to the first file at this point in the
branch; we'll fix this in the next commit.
This test would fail on master for multiple reasons.
