Folding a series of commits into one required either an interactive
rebase where each commit after the first was hand-edited to "fixup", or
a "git reset --soft" to the merge base followed by "git commit --amend".

Add "git rebase --autosquash --fixup [<upstream>]" to do this directly.
It keeps the oldest commit in the range as a "pick" and turns every
later commit into a "fixup", so the whole range collapses into a single
commit that reuses the oldest commit's message. With no <upstream>
argument the range is "@{upstream}..HEAD", folding all unpushed commits
into one.

Fold the commits in their original order, so that any fixup!/squash!
commits already present in the range are folded in as well. Allow the
flag only together with --autosquash.

Signed-off-by: Harald Nordgren <haraldnordgren@gmail.com>