Incorrect parsing of `isBinary`

[jclem]

This shows a diff of a binary file:

$ git diff
diff --git c/image.png w/image.png
index 9be2f56..f30aa31 100644
Binary files c/image.png and w/image.png differ

Note that the first line says "diff --git", which means that gitdiff-parser treats this as a normal file. The "Binary" prefix isn't until later.