module is not pure Go

the current description says:

A Pure Go Git Implementation

this is not true. If I run a diff, everything looks normal:

> dgit diff
diff --git a/README.md b/README.md
--- a/filter/README.md
+++ b/filter/README.md
@@ -4,3 +4,5 @@

 You shouldn't use it either.

+asdf

but then I discovered that it only works because its calling diff from my
path. this is evidenced by it breaking when you clear the path:

> $env:path = ''
> dgit diff
diff --git a/README.md b/README.md

so not only is it not pure Go, but its calling an external executable. and to
top it off, it fails without error.

https://github.com/driusan/dgit/blob/f39f0c15/git/hashdiff.go#L81