Add `revert-file` to your git config, reverting all changes done in a commit to a specific file.

.gitconfig

[alias]
  revert-file = !sh /home/some-user/git-file-revert.sh

git-file-revert.sh

#!/bin/bash

output_help ()
{
    echo "usage: git-revert-single-file <sha1> <file>"
}

sha1=$1
file=$2

if [ $sha1 ]; then
git diff $sha1..$sha1^ -- $file | patch -p1
else
output_help
fi

This was lifted / altered from the following StackOverflow links.

http://stackoverflow.com/a/2620822/23875
http://stackoverflow.com/a/1309674/23875

This is super handy, thank you!