Q:

Convert a two-parent commit into a single-parent one in git

Oops! It seems I did something wrong the last time I merged two branches in my repository:
Original repository http://img532.imageshack.us/img532/9039/screenshotrm.png
This was far from what I was expecting. Is there a way to arrange this mess and obtain something like the following? What did I do wrong?
What I am looking for http://img684.imageshack.us/img684/9977/screenshot2ah.png

A:

A merge commit should have two parents. That's the entire point of a merge.
If you have a really good reason for wanting a merge with only one parent, you can use the --squash option:
# starting with branch-full-reordering at "Corrected GPU float measures"
git merge --squash branch-full-reordering-wo-auxarray-wo-diver
# go on to make the "improve performance commit"

but this is very, very rarely what you want. It dumps all the work done on that other branch into a merge commit, destroying history.
The history you describe, though, isn't a merge at all, squash or otherwise. It's a rebase.
# starting with branch-full-reordering at "Corrected GPU float measures"
git rebase branch-full-reordering branch-full-reordering-wo-auxarray-wo-diver
# go on to make the "improve performance commit"

In this case there will be no merge commit - you've simply transplanted (rebased) the secondary branch onto the primary one.
I should emphasize again: the history you ended up with is likely what you want. It records the fact that some of the development took place on a side branch, then was merged back in.
To revise your history to look the way you want it, you could do something like this:
# recreate your topic branch
git branch topic <SHA1 of commit "Correct pesky bug">
# overkill: you could also specify that as branch-full-reordering^^2
# which means the second parent of the [first] parent of the branch tip

# rebase the topic branch like you meant to in the first place :)
# ~2 means two commits before
# you could also just use the SHA1 of commit "Corrected GPU float measures"
git rebase branch-full-reordering~2 topic

# rebase the main branch onto the topic
# the merge will be ignored, so this will just move the "Improve performance" commit    
git rebase topic branch-full-reordering

# delete the topic branch
git branch -d topic

