Q:

How can I merge a specific file from one branch into another branch in Git

I have 2 branches in a git repo, lets call them, dev and test.  I have changes in a single file, somecode.js.  Both branches have changes to somecode.js.  The 2 branches have diverged significantly (but manageably) so a straight "merge" is insufficient.
I have tried http://jasonrudolph.com/blog/2009/02/25/git-tip-how-to-merge-specific-files-from-another-branch/ but it doesn't merge the contents of both files.  You basically just write over the file instead of actually merging the contents of the file.
I have also tried:
git checkout -b newbranch
git checkout test somecode.js
git commit -m "somecode changes from newbranch"
git checkout dev
git merge newbranch

And
git checkout -m test somecode.js

(I was really hopeful with the -m for merge, but it didn't appear to work for me...)
I thought I was close to what I needed, but then I realized that it just fast-forwarded the commit meaning it didn't merge, it wrote over the original file in test.
So, to reiterate, how can I merge a specific file from one branch into another branch without just writing over the file in the branch I am merging into using git.

A:

I think you like to use
git checkout -p

In your case
git checkout dev
git checkout -p test somecode.js

And you can interactively apply the diffs.

A:

git checkout dev
git show test:somecode.js > somecode.js.theirs
git show $(git merge-base dev test):somecode.js > somecode.js.base
git merge-file somecode.js somecode.js.base somecode.js.theirs 

This way you'll manually make a three-way merge from somecode.js on test branch into somecode.js on dev branch.
Or.. You can create a temporary branch with the changes you want and make a squash merge from it. Is it 'git way' enough? :)
git checkout -b test/filtered $(git merge-base dev test)
git diff ..test -- somecode.js | git apply
git add -- somecode.js
git commit -m "Updated somecode.js"
git checkout dev
git merge --squash test/filtered
git branch -D test/filtered

