Git: Merge old commit into current head version

Question

Got a file that has two commits of interest, both on the Master branch, both only modifying a single file foo: a previous commit AA, and the current version in HEAD. I would like to merge the two versions of the file, keeping bits of both, into HEAD on Master.

I did the simplest thing that I thought would work:

git checkout -b merge-purgatory AA
git commit -m "pulled foo back from previous commit for merging into HEAD."
git checkout master
git merge merge-purgatory

Which simply overwrites the current HEAD version of foo with AA version. Tried the more verbose git checkout -m as well, same result: a dumb overwrite.

How do I force git to treat the AA version of foo as a conflicting merge with the current HEAD version?

Answer 1

If git's merge isn't doing what you want, you could do the following instead:

1. Make sure that there are no uncommitted changes in the file foo, e.g. by making sure that git status is clean.
2. Overwrite foo with the version from AA using: git show AA:foo > foo
3. Selectively stage only the changes from foo that you want with: git add -p foo
4. Discard all the other changes to foo with git checkout -- foo
5. Commit the staged changes: git commit

Alternatively, if you'd rather use a graphical diff tool (such as meld), you could just do:

git show AA:foo > old-foo
meld old-foo foo