Fixing parsing a diff when a file is added w/o content

[bzanchet]

Hi there! I was trying to use the GitDiff library when I came across this bug-- when an empty file is added w/o any content* the patch doesn't have from and to fields with the expected values.

Not sure if including file_a and file_b on the headers is the best solution here but it's the one I could find :)

Please let me know if this works out.

Also-- I noticed some files are not mix format-compliant**. Would be happy to fix that as well (maybe on the same PR?)

Best! Bruno

*like this:

touch file.txt
git add file.txt
git commit -m "adding empty file"

** try:

mix format --check-formatted lib/git_diff.ex 

** (Mix) mix format failed due to --check-formatted.
The following files are not formatted:

  * lib/git_diff.ex