How to revert a file to a known working state and apply upstream changes in a patch from a different branch.

git-merge-single-file.sh

# Check out the original file should receive the changes. This should be the version on your working branch
git checkout {MY-WORKING-BRANCH-SHA} -- path/to/my/file.txt

# Generate a diff of the file, ignoring space changes against a known branch. You should be on your working branch for this
git checkout my-working-branch
git diff HEAD..master --ignore-space-change -- path/to/my/file.txt > patch.diff

# Apply the patch. Note that you MUST use the same --ignore-space-change flag to apply properly
git apply --ignore-space-change patch.diff