Temporary replace "diff-so-fancy" with bundled "diff-highlight"

[arcticicestudio]

Related to so-fancy/diff-so-fancy#35 and so-fancy/diff-so-fancy#296

The reason are incompatibility problems of diff-so-fancy with the latest Git versions caused by changes in the diff logic.

fatal: mismatched output from interactive.diffFilter
hint: Your filter must maintain a one-to-one correspondence
hint: between its input and output lines.

See so-fancy/diff-so-fancy#35 and so-fancy/diff-so-fancy#296 for details.

❗ Downgrading to a compatible Git version is not an option because versions <2.17.1 are effected by CVE-2018-11233 and and 11235.

To fix this problem for interactive patch mode the filter will temporary be changed to the official diff-highlight tool bundled with Git.

[interactive]
-  diffFilter = diff-so-fancy
+  diffFilter = diff-highlight